Use clang on OS X instead of gcc48

This is just to avoid having to install an extra dependency (IIRC gcc takes a while to build). I tested this on OS X 10.9.1, and both the ./install/radiant.bin and the .app bundle produced by apple/Makefile seem to work.

Also - add 'pkgconfig' to the list of MacPorts to install, as it's a required dependency.