FindOpenMP: Add support for Clang on Windows

Fixes: #19404

Modules/FindOpenMP.cmake

@@ -96,7 +96,7 @@ function(_OPENMP_FLAG_CANDIDATES LANG)
     unset(OpenMP_FLAG_CANDIDATES)
 
     set(OMP_FLAG_GNU "-fopenmp")
-    set(OMP_FLAG_Clang "-fopenmp=libomp" "-fopenmp=libiomp5" "-fopenmp")
+    set(OMP_FLAG_Clang "-fopenmp=libomp" "-fopenmp=libiomp5" "-fopenmp" "-Xclang -fopenmp")
     set(OMP_FLAG_AppleClang "-Xclang -fopenmp")
     set(OMP_FLAG_HP "+Oopenmp")
     if(WIN32)
@@ -299,6 +299,25 @@ function(_OPENMP_GET_FLAGS LANG FLAG_MODE OPENMP_FLAG_VAR OPENMP_LIB_NAMES_VAR)
           break()
         endif()
       endif()
+    elseif(CMAKE_${LANG}_COMPILER_ID STREQUAL "Clang" AND WIN32)
+      # Check for separate OpenMP library for Clang on Windows
+      find_library(OpenMP_libomp_LIBRARY
+        NAMES libomp libgomp libiomp5
+        HINTS ${CMAKE_${LANG}_IMPLICIT_LINK_DIRECTORIES}
+      )
+      mark_as_advanced(OpenMP_libomp_LIBRARY)
+      if(OpenMP_libomp_LIBRARY)
+        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} ${OpenMP_libomp_LIBRARY}
+          OUTPUT_VARIABLE OpenMP_TRY_COMPILE_OUTPUT
+        )
+        if(OpenMP_COMPILE_RESULT_${FLAG_MODE}_${OPENMP_PLAIN_FLAG})
+          set("${OPENMP_FLAG_VAR}" "${OPENMP_FLAG}" PARENT_SCOPE)
+          set("${OPENMP_LIB_NAMES_VAR}" "libomp" PARENT_SCOPE)
+          break()
+        endif()
+      endif()
     else()
       file(APPEND ${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/CMakeError.log
         "Detecting ${LANG} OpenMP failed with the following output:\n${OpenMP_TRY_COMPILE_OUTPUT}\n\n")