FindOpenMP: Add support for AppleClang compiler

This is possible using an external `omp` library and by passing
the flag past the compiler driver via `-Xclang -fopenmp`.

Fixes: #17775

Modules/FindOpenMP.cmake

@@ -85,6 +85,7 @@ function(_OPENMP_FLAG_CANDIDATES LANG)
 
     set(OMP_FLAG_GNU "-fopenmp")
     set(OMP_FLAG_Clang "-fopenmp=libomp" "-fopenmp=libiomp5" "-fopenmp")
+    set(OMP_FLAG_AppleClang "-Xclang -fopenmp")
     set(OMP_FLAG_HP "+Oopenmp")
     if(WIN32)
       set(OMP_FLAG_Intel "-Qopenmp")
@@ -256,6 +257,28 @@ function(_OPENMP_GET_FLAGS LANG FLAG_MODE OPENMP_FLAG_VAR OPENMP_LIB_NAMES_VAR)
         endif()
       endif()
       break()
+    elseif(CMAKE_${LANG}_COMPILER_ID STREQUAL "AppleClang"
+      AND CMAKE_${LANG}_COMPILER_VERSION VERSION_GREATER_EQUAL "7.0")
+
+      # Check for separate OpenMP library on AppleClang 7+
+      find_library(OpenMP_libomp_LIBRARY
+        NAMES omp gomp iomp5
+        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()
     endif()
     set("${OPENMP_LIB_NAMES_VAR}" "NOTFOUND" PARENT_SCOPE)
     set("${OPENMP_FLAG_VAR}" "NOTFOUND" PARENT_SCOPE)