Tweak to only override CMAKE_OSX_ARCHITECTURES if its not set
Better handling of default values for CMAKE_OSX_ARCHITECTURES and allowing for override.  Rather than the Xcode default of only building for the current platform, universal binaries are always built.

Use CMAKE_OSX_ARCHITECTURES environment variable to CMake to specify the archs to build, or -DBUILD_UNIVERSAL_BINARIES=OFF argument to cmake to use Xcode default values