Fix header files include in Mac OS X
Fix header files include in Mac OS X

The path of header files of FreeType is: `/opt/local/include/freetype2/freetype`. Without this path `cl-freetype2` will NOT compile success in Mac OS X.

More info related:
  OS: Mac OS X
  FreeType: FreeType v2.6.3(installed with brew)
rpav/cl-freetype2,rpav/cl-freetype2