macOS: add support for OpenMP, making smoke/fluid/cloth simulations faster.

This bring macOS on par with Windows and Linux. It uses the OpenMP library
added to our precompiled libraries.

Custom flags are set because FindOpenMP from CMake below 3.12 does not support
AppleClang, and more recent versions do not work with our custom directory
location either.

Differential Revision: https://developer.blender.org/D4257

source/creator/CMakeLists.txt

@@ -914,6 +914,13 @@ elseif(APPLE)
 		blender.app/Contents/
 	)
 
+	if(WITH_OPENMP AND OPENMP_CUSTOM)
+		install(
+			FILES ${LIBDIR}/openmp/lib/libomp.dylib
+			DESTINATION blender.app/Contents/Resources/lib
+		)
+	endif()
+
 	if(WITH_LLVM AND NOT LLVM_STATIC)
 		install(
 			FILES ${LIBDIR}/llvm/lib/libLLVM-3.4.dylib