cmake: force-disable unity for clang-tidied build targets only

Instead of globally disabling unity for all targets when clang-tidy is
enabled.

After this patch `CMAKE_UNITY_BUILD=ON` is honored for:
- static libcurl when building both static and shared separately.
- libcurlu and libcurltool internal libraries when building the test
  target.

While keeping unity disabled for the libcurl build pass running
clang-tidy, and the curl tool, also running clang-tidy.

To make clang-tidy-enabled builds finish faster when unity mode is
enabled, yet performs the same clang-tidy checks as before this patch.

Effect on:
- GHA/macos: core build: same, buils tests 5-12 seconds faster,
  with steps going down from 259 to 25.

  52s: https://github.com/curl/curl/actions/runs/22279958340/job/64448913325 ->
  47s: https://github.com/curl/curl/actions/runs/22279873606/job/64448710743

- GHA/windows (not enabled): it'd save about 1 minute, bringing total
  time barely below 10m, still one of the slowest jobs overall.
  (#20667 is trying a way for 4x speed-up (with a drawback)).

  5m21s: https://github.com/curl/curl/actions/runs/22222907068/job/64284556852 ->
  4m26s: https://github.com/curl/curl/actions/runs/22281033369/job/64451601548

Closes #20670

CMakeLists.txt

@@ -273,7 +273,6 @@ if(CURL_CLANG_TIDY)
     set(PICKY_COMPILER OFF)  # Do a best effort and skip passing non-clang warning options to clang-tidy.
     # This lets through warning options enabled via CURL_WERROR=ON, affecting lib and src.
   endif()
-  set(CMAKE_UNITY_BUILD OFF)  # clang-tidy is not looking into #included sources, thus not compatible with unity builds.
   set(CURL_DISABLE_TYPECHECK ON)  # to improve performance and avoid potential interference.
   set(CMAKE_C_CLANG_TIDY "${CLANG_TIDY}")
   list(APPEND CMAKE_C_CLANG_TIDY "--config-file=${PROJECT_SOURCE_DIR}/.clang-tidy.yml")

docs/INSTALL-CMAKE.md

@@ -229,7 +229,8 @@ target_link_libraries(my_target PRIVATE CURL::libcurl)
                                             Set `QUICK` to build examples quickly with the `curl-examples-build` target (for build tests).
                                             Set `NOEXAMPLES` to not build examples.
 - `CURL_CLANG_TIDY`:                        Run the build through `clang-tidy`. Default: `OFF`
-                                            If enabled, it implies `CMAKE_UNITY_BUILD=OFF` and `CURL_DISABLE_TYPECHECK=ON`.
+                                            If enabled, it implies `CURL_DISABLE_TYPECHECK=ON` and force-disables unity mode
+                                            for libcurl and the curl tool.
 - `CURL_CLANG_TIDYFLAGS`:                   Custom options to pass to `clang-tidy`. Default: (empty)
 - `CURL_CODE_COVERAGE`:                     Enable code coverage build options. Default: `OFF`
 - `CURL_COMPLETION_FISH`:                   Install fish completions. Default: `OFF`

lib/CMakeLists.txt

@@ -125,6 +125,9 @@ if(SHARE_LIB_OBJECT AND CMAKE_VERSION VERSION_GREATER_EQUAL 3.12)
       set_target_properties(${LIB_OBJECT} PROPERTIES INTERPROCEDURAL_OPTIMIZATION TRUE)
     endif()
   endif()
+  if(CURL_CLANG_TIDY)
+    set_target_properties(${LIB_OBJECT} PROPERTIES UNITY_BUILD OFF)
+  endif()
   if(CURL_CODE_COVERAGE)
     set_property(TARGET ${LIB_OBJECT} APPEND PROPERTY COMPILE_DEFINITIONS ${CURL_COVERAGE_MACROS})
     set_property(TARGET ${LIB_OBJECT} APPEND PROPERTY COMPILE_OPTIONS ${CURL_COVERAGE_CFLAGS})
@@ -167,8 +170,12 @@ if(BUILD_STATIC_LIBS)
       set_target_properties(${LIB_STATIC} PROPERTIES INTERPROCEDURAL_OPTIMIZATION TRUE)
     endif()
   endif()
-  if(CURL_CLANG_TIDY AND BUILD_SHARED_LIBS)  # disable clang-tidy for static, and limit to the shared library, if both enabled
-    set_target_properties(${LIB_STATIC} PROPERTIES C_CLANG_TIDY "")
+  if(CURL_CLANG_TIDY)
+    if(BUILD_SHARED_LIBS)  # disable clang-tidy for static, and limit to the shared library, if both enabled
+      set_target_properties(${LIB_STATIC} PROPERTIES C_CLANG_TIDY "")
+    else()
+      set_target_properties(${LIB_STATIC} PROPERTIES UNITY_BUILD OFF)
+    endif()
   endif()
   if(CURL_CODE_COVERAGE)
     set_property(TARGET ${LIB_STATIC} APPEND PROPERTY COMPILE_DEFINITIONS ${CURL_COVERAGE_MACROS})
@@ -232,6 +239,9 @@ if(BUILD_SHARED_LIBS)
       set_target_properties(${LIB_SHARED} PROPERTIES INTERPROCEDURAL_OPTIMIZATION TRUE)
     endif()
   endif()
+  if(CURL_CLANG_TIDY)
+    set_target_properties(${LIB_SHARED} PROPERTIES UNITY_BUILD OFF)
+  endif()
   if(CURL_CODE_COVERAGE)
     set_property(TARGET ${LIB_SHARED} APPEND PROPERTY COMPILE_DEFINITIONS ${CURL_COVERAGE_MACROS})
     set_property(TARGET ${LIB_SHARED} APPEND PROPERTY COMPILE_OPTIONS ${CURL_COVERAGE_CFLAGS})

src/CMakeLists.txt

@@ -121,6 +121,9 @@ set_target_properties(curltool PROPERTIES C_CLANG_TIDY "")
 if(CURL_HAS_LTO)
   set_target_properties(${EXE_NAME} PROPERTIES INTERPROCEDURAL_OPTIMIZATION TRUE)
 endif()
+if(CURL_CLANG_TIDY)
+  set_target_properties(${EXE_NAME} PROPERTIES UNITY_BUILD OFF)
+endif()
 
 if(ENABLE_UNICODE AND MINGW)
   if(CMAKE_VERSION VERSION_GREATER_EQUAL 3.13)