FindOpenMP: Use OpenMP_<lang>_INCLUDE_DIR

If `OpenMP_<lang>_INCLUDE_DIR` is defined, add it to the list of include
directories before checking flags. Previously, this variable was
ignored for all compilers but AppleClang, despite the documentation
mentioning it as one of the possible inputs.

Fixes: #24260

Modules/FindOpenMP.cmake

@@ -212,10 +212,14 @@ function(_OPENMP_GET_FLAGS LANG FLAG_MODE OPENMP_FLAG_VAR OPENMP_LIB_NAMES_VAR)
       string(APPEND OPENMP_FLAGS_TEST " ${OpenMP_VERBOSE_COMPILE_OPTIONS}")
     endif()
     string(REGEX REPLACE "[-/=+]" "" OPENMP_PLAIN_FLAG "${OPENMP_FLAG}")
+    unset(_includeDirFlags)
+    if(OpenMP_${LANG}_INCLUDE_DIR)
+      set(_includeDirFlags "-DINCLUDE_DIRECTORIES:STRING=${OpenMP_${LANG}_INCLUDE_DIR}")
+    endif()
     try_compile( OpenMP_COMPILE_RESULT_${FLAG_MODE}_${OPENMP_PLAIN_FLAG}
       SOURCE_FROM_VAR "${_OPENMP_TEST_SRC_NAME}" _OPENMP_TEST_SRC_CONTENT
       LOG_DESCRIPTION "Detecting ${LANG} OpenMP compiler info"
-      CMAKE_FLAGS "-DCOMPILE_DEFINITIONS:STRING=${OPENMP_FLAGS_TEST}"
+      CMAKE_FLAGS "-DCOMPILE_DEFINITIONS:STRING=${OPENMP_FLAGS_TEST}" ${_includeDirFlags}
       LINK_LIBRARIES ${CMAKE_${LANG}_VERBOSE_FLAG}
       OUTPUT_VARIABLE OpenMP_TRY_COMPILE_OUTPUT
     )