Support universal builds for Mac OSX

When building on Darwin (Mac OSX), allow the creating of a universal binary and create header files that can be used on several architectures.  This is achieved by looking at the preprocessor symbols __ppc__, __ppc64__, __i386__, and __x86_64__ which are always defined according to documentation from Apple, and the creation of a special header file libffitarget.h in directory src/universal, that is assembled from src/powerpc/libffitarget.h and sr/x86/libffitarget.h.

Tested with the gtk-osx framework.