Two changes:
Removed the explicit version for the macOS SDK, recent
versions of Xcode have a symlink to the newest SDK.
Fixed the build script for OpenMP by removing extra ' marks that
install_name_tool took literally and replaced INSTALL_PATH with
INSTALL_DIR.
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
