BUILD: prefer pkg-config for freetype2 detection

This can solve numerous problems such as:
http://www.cmake.org/Bug/view.php?id=13959
http://www.cmake.org/Bug/view.php?id=14601

If pkg-config or freetype2.pc is not found, then fall back to the
FindFreetype.cmake module logic.

Restrict to UNIX since I only tested it here.