cmake: add CMake Config-based dependency detection

After limiting `find_package()`/`find_dependency()` calls to curl local
Find modules via the `MODULES` keyword, it became possible to detect
dependencies via CMake Configs from within those local Find modules, by
calling `find_package()` again with the `CONFIG` keyword. This patch
implements this. Then maps detection results to the result variables and
curl-specific imported targets the rest of the build expects.

Also honor recently introduced `*_USE_STATIC_LIBS` (experimental) flags
to map to the static target when requested.

This adds CMake Configs as an alternative to the existing `pkg-config`
and `find_path()`/`find_library()` auto-detection methods.

Enabled by default for MSVC, outside vcpkg and when not cross-building.
To enable for other cases, or override the default, you can use
`-DCURL_USE_CMAKECONFIG=ON` or `OFF`.

When enabled, Config detection happens after `pkg-config` and before
`find_path()`/`find_library()`. Using CMake's built-in options, you may
also manually point to the absolute directory holding Config files:

`Libssh2_DIR`, `MbedTLS_DIR`, `NGHTTP2_DIR`, `NGHTTP3_DIR`,
`NGTCP2_DIR` v1.19.0+ (with non-fork OpenSSL only), `Zstd_DIR` v1.4.5+

E.g. `-DMbedTLS_DIR=/path/to/mbedtls/lib/cmake/MbedTLS`

These dependencies typically need to be built with CMake to support
this.

Tagged as experimental.

Refs:
#20013 #19156 #19117
https://github.com/curl/curl/pull/20784#issuecomment-3984318492

Depends-on: fad1ebaecc #20840
Follow-up to 91e06fde1b #20784
Follow-up to 26c39d8df1 #20015

Closes #20814

CMake/FindLibssh2.cmake

@@ -37,11 +37,15 @@
 
 set(_libssh2_pc_requires "libssh2")
 
-if(CURL_USE_PKGCONFIG AND
-   NOT DEFINED LIBSSH2_INCLUDE_DIR AND
+if(NOT DEFINED LIBSSH2_INCLUDE_DIR AND
    NOT DEFINED LIBSSH2_LIBRARY)
-  find_package(PkgConfig QUIET)
-  pkg_check_modules(_libssh2 ${_libssh2_pc_requires})
+  if(CURL_USE_PKGCONFIG)
+    find_package(PkgConfig QUIET)
+    pkg_check_modules(_libssh2 ${_libssh2_pc_requires})
+  endif()
+  if(NOT _libssh2_FOUND AND CURL_USE_CMAKECONFIG)
+    find_package(libssh2 CONFIG QUIET)
+  endif()
 endif()
 
 if(_libssh2_FOUND AND _libssh2_INCLUDE_DIRS)
@@ -55,6 +59,16 @@ if(_libssh2_FOUND AND _libssh2_INCLUDE_DIRS)
     set(_libssh2_LIBRARIES    "${_libssh2_STATIC_LIBRARIES}")
   endif()
   message(STATUS "Found Libssh2 (via pkg-config): ${_libssh2_INCLUDE_DIRS} (found version \"${LIBSSH2_VERSION}\")")
+elseif(libssh2_CONFIG)
+  set(Libssh2_FOUND TRUE)
+  set(LIBSSH2_FOUND TRUE)
+  set(LIBSSH2_VERSION ${libssh2_VERSION})
+  if(LIBSSH2_USE_STATIC_LIBS)
+    set(_libssh2_LIBRARIES libssh2::libssh2_static)
+  else()
+    set(_libssh2_LIBRARIES libssh2::libssh2)
+  endif()
+  message(STATUS "Found Libssh2 (via CMake Config): ${libssh2_CONFIG} (found version \"${LIBSSH2_VERSION}\")")
 else()
   find_path(LIBSSH2_INCLUDE_DIR NAMES "libssh2.h")
   if(LIBSSH2_USE_STATIC_LIBS)

CMake/FindMbedTLS.cmake

@@ -45,13 +45,17 @@ endif()
 
 set(_mbedtls_pc_requires "mbedtls" "mbedx509" "mbedcrypto")
 
-if(CURL_USE_PKGCONFIG AND
-   NOT DEFINED MBEDTLS_INCLUDE_DIR AND
+if(NOT DEFINED MBEDTLS_INCLUDE_DIR AND
    NOT DEFINED MBEDTLS_LIBRARY AND
    NOT DEFINED MBEDX509_LIBRARY AND
    NOT DEFINED MBEDCRYPTO_LIBRARY)
-  find_package(PkgConfig QUIET)
-  pkg_check_modules(_mbedtls ${_mbedtls_pc_requires})
+  if(CURL_USE_PKGCONFIG)
+    find_package(PkgConfig QUIET)
+    pkg_check_modules(_mbedtls ${_mbedtls_pc_requires})
+  endif()
+  if(NOT _mbedtls_FOUND AND CURL_USE_CMAKECONFIG)
+    find_package(MbedTLS CONFIG QUIET)
+  endif()
 endif()
 
 if(_mbedtls_FOUND)
@@ -65,6 +69,17 @@ if(_mbedtls_FOUND)
     set(_mbedtls_LIBRARIES    "${_mbedtls_STATIC_LIBRARIES}")
   endif()
   message(STATUS "Found MbedTLS (via pkg-config): ${_mbedtls_INCLUDE_DIRS} (found version \"${MBEDTLS_VERSION}\")")
+elseif(MbedTLS_CONFIG)
+  set(MbedTLS_FOUND TRUE)
+  set(MBEDTLS_FOUND TRUE)
+  set(MBEDTLS_VERSION ${MbedTLS_VERSION})
+  if(MBEDTLS_VERSION GREATER_EQUAL 4.0.0)
+    set(_mbedtls_LIBRARIES MbedTLS::tfpsacrypto)
+  else()
+    set(_mbedtls_LIBRARIES MbedTLS::mbedcrypto)
+  endif()
+  list(APPEND _mbedtls_LIBRARIES MbedTLS::mbedx509 MbedTLS::mbedtls)
+  message(STATUS "Found MbedTLS (via CMake Config): ${MbedTLS_CONFIG} (found version \"${MBEDTLS_VERSION}\")")
 else()
   set(_mbedtls_pc_requires "")  # Depend on pkg-config only when found via pkg-config
 

CMake/FindNGHTTP2.cmake

@@ -37,11 +37,15 @@
 
 set(_nghttp2_pc_requires "libnghttp2")
 
-if(CURL_USE_PKGCONFIG AND
-   NOT DEFINED NGHTTP2_INCLUDE_DIR AND
+if(NOT DEFINED NGHTTP2_INCLUDE_DIR AND
    NOT DEFINED NGHTTP2_LIBRARY)
-  find_package(PkgConfig QUIET)
-  pkg_check_modules(_nghttp2 ${_nghttp2_pc_requires})
+  if(CURL_USE_PKGCONFIG)
+    find_package(PkgConfig QUIET)
+    pkg_check_modules(_nghttp2 ${_nghttp2_pc_requires})
+  endif()
+  if(NOT _nghttp2_FOUND AND CURL_USE_CMAKECONFIG)
+    find_package(nghttp2 CONFIG QUIET)
+  endif()
 endif()
 
 if(_nghttp2_FOUND)
@@ -54,6 +58,15 @@ if(_nghttp2_FOUND)
     set(_nghttp2_LIBRARIES    "${_nghttp2_STATIC_LIBRARIES}")
   endif()
   message(STATUS "Found NGHTTP2 (via pkg-config): ${_nghttp2_INCLUDE_DIRS} (found version \"${NGHTTP2_VERSION}\")")
+elseif(nghttp2_CONFIG)
+  set(NGHTTP2_FOUND TRUE)
+  set(NGHTTP2_VERSION ${nghttp2_VERSION})
+  if(NGHTTP2_USE_STATIC_LIBS)
+    set(_nghttp2_LIBRARIES nghttp2::nghttp2_static)
+  else()
+    set(_nghttp2_LIBRARIES nghttp2::nghttp2)
+  endif()
+  message(STATUS "Found NGHTTP2 (via CMake Config): ${nghttp2_CONFIG} (found version \"${NGHTTP2_VERSION}\")")
 else()
   find_path(NGHTTP2_INCLUDE_DIR NAMES "nghttp2/nghttp2.h")
   if(NGHTTP2_USE_STATIC_LIBS)

CMake/FindNGHTTP3.cmake

@@ -37,11 +37,15 @@
 
 set(_nghttp3_pc_requires "libnghttp3")
 
-if(CURL_USE_PKGCONFIG AND
-   NOT DEFINED NGHTTP3_INCLUDE_DIR AND
+if(NOT DEFINED NGHTTP3_INCLUDE_DIR AND
    NOT DEFINED NGHTTP3_LIBRARY)
-  find_package(PkgConfig QUIET)
-  pkg_check_modules(_nghttp3 ${_nghttp3_pc_requires})
+  if(CURL_USE_PKGCONFIG)
+    find_package(PkgConfig QUIET)
+    pkg_check_modules(_nghttp3 ${_nghttp3_pc_requires})
+  endif()
+  if(NOT _nghttp3_FOUND AND CURL_USE_CMAKECONFIG)
+    find_package(nghttp3 CONFIG QUIET)
+  endif()
 endif()
 
 if(_nghttp3_FOUND)
@@ -54,6 +58,15 @@ if(_nghttp3_FOUND)
     set(_nghttp3_LIBRARIES    "${_nghttp3_STATIC_LIBRARIES}")
   endif()
   message(STATUS "Found NGHTTP3 (via pkg-config): ${_nghttp3_INCLUDE_DIRS} (found version \"${NGHTTP3_VERSION}\")")
+elseif(nghttp3_CONFIG)
+  set(NGHTTP3_FOUND TRUE)
+  set(NGHTTP3_VERSION ${nghttp3_VERSION})
+  if(NGHTTP3_USE_STATIC_LIBS)
+    set(_nghttp3_LIBRARIES nghttp3::nghttp3_static)
+  else()
+    set(_nghttp3_LIBRARIES nghttp3::nghttp3)
+  endif()
+  message(STATUS "Found NGHTTP3 (via CMake Config): ${nghttp3_CONFIG} (found version \"${NGHTTP3_VERSION}\")")
 else()
   find_path(NGHTTP3_INCLUDE_DIR NAMES "nghttp3/nghttp3.h")
   if(NGHTTP3_USE_STATIC_LIBS)

CMake/FindNGTCP2.cmake

@@ -74,12 +74,22 @@ if(_ngtcp2_crypto_backend)
 endif()
 
 set(_tried_pkgconfig FALSE)
-if(CURL_USE_PKGCONFIG AND
-   NOT DEFINED NGTCP2_INCLUDE_DIR AND
+if(NOT DEFINED NGTCP2_INCLUDE_DIR AND
    NOT DEFINED NGTCP2_LIBRARY)
-  find_package(PkgConfig QUIET)
-  pkg_check_modules(_ngtcp2 ${_ngtcp2_pc_requires})
-  set(_tried_pkgconfig TRUE)
+  if(CURL_USE_PKGCONFIG)
+    find_package(PkgConfig QUIET)
+    pkg_check_modules(_ngtcp2 ${_ngtcp2_pc_requires})
+    set(_tried_pkgconfig TRUE)
+  endif()
+  if(NOT _ngtcp2_FOUND AND CURL_USE_CMAKECONFIG)
+    find_package(ngtcp2 CONFIG QUIET)
+    # Skip using it if the crypto library target is not available
+    if(ngtcp2_CONFIG AND
+       NOT TARGET ngtcp2::${_crypto_library_lower}_static AND
+       NOT TARGET ngtcp2::${_crypto_library_lower})
+      unset(ngtcp2_CONFIG)
+    endif()
+  endif()
 endif()
 
 if(_ngtcp2_FOUND)
@@ -92,6 +102,15 @@ if(_ngtcp2_FOUND)
     set(_ngtcp2_LIBRARIES    "${_ngtcp2_STATIC_LIBRARIES}")
   endif()
   message(STATUS "Found NGTCP2 (via pkg-config): ${_ngtcp2_INCLUDE_DIRS} (found version \"${NGTCP2_VERSION}\")")
+elseif(ngtcp2_CONFIG)
+  set(NGTCP2_FOUND TRUE)
+  set(NGTCP2_VERSION ${ngtcp2_VERSION})
+  if(NGTCP2_USE_STATIC_LIBS)
+    set(_ngtcp2_LIBRARIES ngtcp2::ngtcp2_static ngtcp2::${_crypto_library_lower}_static)
+  else()
+    set(_ngtcp2_LIBRARIES ngtcp2::ngtcp2 ngtcp2::${_crypto_library_lower})
+  endif()
+  message(STATUS "Found NGTCP2 (via CMake Config): ${ngtcp2_CONFIG} (found version \"${NGTCP2_VERSION}\")")
 else()
   find_path(NGTCP2_INCLUDE_DIR NAMES "ngtcp2/ngtcp2.h")
   if(NGTCP2_USE_STATIC_LIBS)

CMake/FindZstd.cmake

@@ -46,11 +46,21 @@ endif()
 
 set(_zstd_pc_requires "libzstd")
 
-if(CURL_USE_PKGCONFIG AND
-   NOT DEFINED ZSTD_INCLUDE_DIR AND
+if(NOT DEFINED ZSTD_INCLUDE_DIR AND
    NOT DEFINED ZSTD_LIBRARY)
-  find_package(PkgConfig QUIET)
-  pkg_check_modules(_zstd ${_zstd_pc_requires})
+  if(CURL_USE_PKGCONFIG)
+    find_package(PkgConfig QUIET)
+    pkg_check_modules(_zstd ${_zstd_pc_requires})
+  endif()
+  if(NOT _zstd_FOUND AND CURL_USE_CMAKECONFIG)
+    find_package(Zstd CONFIG QUIET)
+    # Skip using if older than v1.4.5
+    if(Zstd_CONFIG AND
+       NOT TARGET zstd::libzstd_static AND
+       NOT TARGET zstd::libzstd_shared)
+      unset(Zstd_CONFIG)
+    endif()
+  endif()
 endif()
 
 if(_zstd_FOUND)
@@ -64,6 +74,17 @@ if(_zstd_FOUND)
     set(_zstd_LIBRARIES    "${_zstd_STATIC_LIBRARIES}")
   endif()
   message(STATUS "Found Zstd (via pkg-config): ${_zstd_INCLUDE_DIRS} (found version \"${ZSTD_VERSION}\")")
+elseif(Zstd_CONFIG)
+  set(ZSTD_FOUND TRUE)
+  set(ZSTD_VERSION ${Zstd_VERSION})
+  if(ZSTD_USE_STATIC_LIBS)
+    set(_zstd_LIBRARIES zstd::libzstd_static)
+  elseif(TARGET zstd::libzstd)
+    set(_zstd_LIBRARIES zstd::libzstd)  # v1.5.6+
+  else()
+    set(_zstd_LIBRARIES zstd::libzstd_shared)
+  endif()
+  message(STATUS "Found Zstd (via CMake Config): ${Zstd_CONFIG} (found version \"${ZSTD_VERSION}\")")
 else()
   find_path(ZSTD_INCLUDE_DIR NAMES "zstd.h")
   if(ZSTD_USE_STATIC_LIBS)

CMake/curl-config.in.cmake

@@ -23,6 +23,8 @@
 ###########################################################################
 @PACKAGE_INIT@
 
+option(CURL_USE_CMAKECONFIG "Enable detecting @PROJECT_NAME@ dependencies via CMake Config. Default: @CURL_USE_CMAKECONFIG@"
+  "@CURL_USE_CMAKECONFIG@")
 option(CURL_USE_PKGCONFIG "Enable pkg-config to detect @PROJECT_NAME@ dependencies. Default: @CURL_USE_PKGCONFIG@"
   "@CURL_USE_PKGCONFIG@")