[Commits] SDL_ttf [and friends]: Use xcrun to find CpMac

The /usr/bin/CpMac command does not support the same options as
the POSIX cp command, and is much less flexible in its operands.
It cannot be used as a direct substitute for cp in scripts.

  As of Mac OS X 10.4, the cp command preserves metadata and re-
  source forks of files on Extended HFS volumes, so it can be used
  in place of CpMac.  The /usr/bin/CpMac command will be deprecated
  in future versions of Mac OS X.

We don’t support 10.4. And while it is possible that someone may
have installed GNU cp, and I am not sure if or at what point GNU cp
gained the ability to handle OS X metadata, but /bin/cp always does
now, on any machine currently supported.

Just a note for future-proofing.