cmake: allow pkg-config in more envs

Before this patch, `pkg-config` was used for `UNIX` builds only (with
a few exceptions like wolfSSL, libssh, gsasl, libuv). This patch extends
`pkg-config` use to all envs except: `MSVC` without vcpkg. Meaning MSVC
with vcpkg will now use it. Also mingw on Windows.

Also apply the new condition to options where `pkg-config` was used
unconditionally (= for all targets). These are:
`-DCURL_USE_WOLFSSL=ON`, `-DCURL_USE_LIBSSH=ON`,
`-DCURL_USE_GSASL=ON` and `-DCURL_USE_LIBUV=ON`

This patch may still cause regressions for cross-builds (e.g. mingw
cross-build from Unix) and potentially other cases. If that happens, we
recommend using some of these methods to explicitly disable `pkg-config`
when using CMake:
- CMake option: `-DPKG_CONFIG_EXECUTABLE=`
  (or `-DPKG_CONFIG_EXECUTABLE=nonexistent` or similar)
  This is similar to the (curl-specific) `PKG_CONFIG` env for autotools.
- export env: `PKG_CONFIG_LIBDIR=`
  (or `PKG_CONFIG_PATH`, `PKG_CONFIG_SYSROOT_DIR`,
  or the CMake-specific `PKG_CONFIG`)

We may improve control over this in a future patch, also allowing opting
in MSVC (without vcpkg).

Ref: #14405
Ref: #14408
Ref: #14140
Closes #14483

CMake/FindNGTCP2.cmake

@@ -38,7 +38,7 @@
 # NGTCP2_LIBRARIES     The libraries needed to use ngtcp2
 # NGTCP2_VERSION       Version of ngtcp2
 
-if(UNIX)
+if(NOT MSVC OR VCPKG_TOOLCHAIN)
   find_package(PkgConfig QUIET)
   pkg_search_module(PC_NGTCP2 "libngtcp2")
 endif()
@@ -72,7 +72,7 @@ if(NGTCP2_FIND_COMPONENTS)
 
   if(NGTCP2_CRYPTO_BACKEND)
     string(TOLOWER "ngtcp2_crypto_${NGTCP2_CRYPTO_BACKEND}" _crypto_library)
-    if(UNIX)
+    if(NOT MSVC OR VCPKG_TOOLCHAIN)
       pkg_search_module(PC_${_crypto_library} "lib${_crypto_library}")
     endif()
     find_library(${_crypto_library}_LIBRARY