FindOpenMP: Handle non-Apple clang on macOS

I was noticing that FindOpenMP was failing to find my openmp installation
on macos when building with a custom-compiled clang. Upon inspection it
appears that FindOpenMP is looking for `AppleClang` as a compiler
identification and the `Clang` case is only applicable for Win32.
Try to fix that by treating custom `Clang` on macos the same as
`AppleClang`.

Modules/FindOpenMP.cmake

@@ -298,8 +298,9 @@ function(_OPENMP_GET_FLAGS LANG FLAG_MODE OPENMP_FLAG_VAR OPENMP_LIB_NAMES_VAR)
         set("${OPENMP_LIB_NAMES_VAR}" "" PARENT_SCOPE)
       endif()
       break()
-    elseif(CMAKE_${LANG}_COMPILER_ID STREQUAL "AppleClang"
-      AND CMAKE_${LANG}_COMPILER_VERSION VERSION_GREATER_EQUAL "7.0")
+    elseif((CMAKE_${LANG}_COMPILER_ID STREQUAL "AppleClang"
+      AND CMAKE_${LANG}_COMPILER_VERSION VERSION_GREATER_EQUAL "7.0") OR
+      (CMAKE_${LANG}_COMPILER_ID STREQUAL "Clang" AND APPLE))
 
       # Check for separate OpenMP library on AppleClang 7+
       find_library(OpenMP_libomp_LIBRARY