FindOpenMP: Allow try_compile() to find omp.h for AppleClang

Fixes: #18098 #18470 #18520

Modules/FindOpenMP.cmake

@@ -36,6 +36,9 @@ project, where ``<lang>`` is one of C, CXX, or Fortran:
   Variable indicating if OpenMP support for ``<lang>`` was detected.
 ``OpenMP_<lang>_FLAGS``
   OpenMP compiler flags for ``<lang>``, separated by spaces.
+``OpenMP_<lang>_INCLUDE_DIRS``
+  Directories that must be added to the header search path for ``<lang>``
+  when using OpenMP.
 
 For linking with OpenMP code written in ``<lang>``, the following
 variables are provided:
@@ -73,6 +76,14 @@ The module will also try to provide the OpenMP version variables:
 The specification date is formatted as given in the OpenMP standard:
 ``yyyymm`` where ``yyyy`` and ``mm`` represents the year and month of
 the OpenMP specification implemented by the ``<lang>`` compiler.
+
+For some compilers, it may be necessary to add a header search path to find
+the relevant OpenMP headers.  This location may be language-specific.  Where
+this is needed, the module may attempt to find the location, but it can be
+provided directly by setting the ``OpenMP_<lang>_INCLUDE_DIR`` cache variable.
+Note that this variable is an _input_ control to the module.  Project code
+should use the ``OpenMP_<lang>_INCLUDE_DIRS`` _output_ variable if it needs
+to know what include directories are needed.
 #]=======================================================================]
 
 cmake_policy(PUSH)
@@ -259,11 +270,27 @@ function(_OPENMP_GET_FLAGS LANG FLAG_MODE OPENMP_FLAG_VAR OPENMP_LIB_NAMES_VAR)
       mark_as_advanced(OpenMP_libomp_LIBRARY)
 
       if(OpenMP_libomp_LIBRARY)
+        # Try without specifying include directory first. We only want to
+        # explicitly add a search path if the header can't be found on the
+        # default header search path already.
         try_compile( OpenMP_COMPILE_RESULT_${FLAG_MODE}_${OPENMP_PLAIN_FLAG} ${CMAKE_BINARY_DIR} ${_OPENMP_TEST_SRC}
           CMAKE_FLAGS "-DCOMPILE_DEFINITIONS:STRING=${OPENMP_FLAGS_TEST}"
           LINK_LIBRARIES ${CMAKE_${LANG}_VERBOSE_FLAG} ${OpenMP_libomp_LIBRARY}
           OUTPUT_VARIABLE OpenMP_TRY_COMPILE_OUTPUT
         )
+        if(NOT OpenMP_COMPILE_RESULT_${FLAG_MODE}_${OPENMP_PLAIN_FLAG})
+          find_path(OpenMP_${LANG}_INCLUDE_DIR omp.h)
+          mark_as_advanced(OpenMP_${LANG}_INCLUDE_DIR)
+          set(OpenMP_${LANG}_INCLUDE_DIR "${OpenMP_${LANG}_INCLUDE_DIR}" PARENT_SCOPE)
+          if(OpenMP_${LANG}_INCLUDE_DIR)
+            try_compile( OpenMP_COMPILE_RESULT_${FLAG_MODE}_${OPENMP_PLAIN_FLAG} ${CMAKE_BINARY_DIR} ${_OPENMP_TEST_SRC}
+              CMAKE_FLAGS "-DCOMPILE_DEFINITIONS:STRING=${OPENMP_FLAGS_TEST}"
+                          "-DINCLUDE_DIRECTORIES:STRING=${OpenMP_${LANG}_INCLUDE_DIR}"
+              LINK_LIBRARIES ${CMAKE_${LANG}_VERBOSE_FLAG} ${OpenMP_libomp_LIBRARY}
+              OUTPUT_VARIABLE OpenMP_TRY_COMPILE_OUTPUT
+            )
+          endif()
+        endif()
         if(OpenMP_COMPILE_RESULT_${FLAG_MODE}_${OPENMP_PLAIN_FLAG})
           set("${OPENMP_FLAG_VAR}" "${OPENMP_FLAG}" PARENT_SCOPE)
           set("${OPENMP_LIB_NAMES_VAR}" "libomp" PARENT_SCOPE)
@@ -323,10 +350,15 @@ set(OpenMP_Fortran_CHECK_VERSION_SOURCE
 function(_OPENMP_GET_SPEC_DATE LANG SPEC_DATE)
   _OPENMP_WRITE_SOURCE_FILE("${LANG}" "CHECK_VERSION_SOURCE" OpenMPCheckVersion _OPENMP_TEST_SRC)
 
+  unset(_includeDirFlags)
+  if(OpenMP_${LANG}_INCLUDE_DIR)
+    set(_includeDirFlags "-DINCLUDE_DIRECTORIES:STRING=${OpenMP_${LANG}_INCLUDE_DIR}")
+  endif()
+
   set(BIN_FILE "${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/FindOpenMP/ompver_${LANG}.bin")
   string(REGEX REPLACE "[-/=+]" "" OPENMP_PLAIN_FLAG "${OPENMP_FLAG}")
   try_compile(OpenMP_SPECTEST_${LANG}_${OPENMP_PLAIN_FLAG} "${CMAKE_BINARY_DIR}" "${_OPENMP_TEST_SRC}"
-              CMAKE_FLAGS "-DCOMPILE_DEFINITIONS:STRING=${OpenMP_${LANG}_FLAGS}"
+              CMAKE_FLAGS "-DCOMPILE_DEFINITIONS:STRING=${OpenMP_${LANG}_FLAGS}" ${_includeDirFlags}
               COPY_FILE ${BIN_FILE}
               OUTPUT_VARIABLE OpenMP_TRY_COMPILE_OUTPUT)
 
@@ -489,6 +521,11 @@ foreach(LANG IN LISTS OpenMP_FINDLIST)
       foreach(_OPENMP_IMPLICIT_LIB IN LISTS OpenMP_${LANG}_LIB_NAMES)
         list(APPEND OpenMP_${LANG}_LIBRARIES "${OpenMP_${_OPENMP_IMPLICIT_LIB}_LIBRARY}")
       endforeach()
+      if(OpenMP_${LANG}_INCLUDE_DIR)
+        set(OpenMP_${LANG}_INCLUDE_DIRS ${OpenMP_${LANG}_INCLUDE_DIR})
+      else()
+        set(OpenMP_${LANG}_INCLUDE_DIRS "")
+      endif()
 
       if(NOT TARGET OpenMP::OpenMP_${LANG})
         add_library(OpenMP::OpenMP_${LANG} INTERFACE IMPORTED)
@@ -499,6 +536,10 @@ foreach(LANG IN LISTS OpenMP_FINDLIST)
           INTERFACE_COMPILE_OPTIONS "$<$<COMPILE_LANGUAGE:${LANG}>:${_OpenMP_${LANG}_OPTIONS}>")
         unset(_OpenMP_${LANG}_OPTIONS)
       endif()
+      if(OpenMP_${LANG}_INCLUDE_DIRS)
+        set_property(TARGET OpenMP::OpenMP_${LANG} PROPERTY
+          INTERFACE_INCLUDE_DIRECTORIES "$<BUILD_INTERFACE:${OpenMP_${LANG}_INCLUDE_DIRS}>")
+      endif()
       if(OpenMP_${LANG}_LIBRARIES)
         set_property(TARGET OpenMP::OpenMP_${LANG} PROPERTY
           INTERFACE_LINK_LIBRARIES "${OpenMP_${LANG}_LIBRARIES}")

Tests/FindOpenMP/Test/CMakeLists.txt

@@ -42,6 +42,7 @@ foreach(c C CXX Fortran)
   separate_arguments(_OpenMP_${c}_OPTIONS NATIVE_COMMAND "${OpenMP_${c}_FLAGS}")
   target_compile_options(test_var_${c} PRIVATE "${_OpenMP_${c}_OPTIONS}")
   target_link_libraries(test_var_${c} PRIVATE "${OpenMP_${c}_FLAGS}")
+  target_include_directories(test_var_${c} PRIVATE ${OpenMP_${c}_INCLUDE_DIRS})
   set_property(TARGET test_var_${c} PROPERTY LINKER_LANGUAGE ${c})
   add_test(NAME test_var_${c} COMMAND test_var_${c})