Added a dupe of libffi that works for linux.

Fixed
=====
* On a 64 bit linux system, the lib files are divided into lib and lib64
  folders. This dupe adjusts the installed libffi.pc to reflect this.
Only add -lintl if on a mac.

Changed
=======
* gettext and libintl are provided by the system glibc for linux so
  don't need to include them.