OS X Makefile build setup

This merge request:

• Cleans up the OS X bundle resource location code and fixes a SIGSEGV and memory leak
• Simplifies and fixes the OS X desktop alert code, closing more leaks
• Adds the MACOSX build flag to the Makefiles, to allow building a binary (but not Mac app yet) of SRB2.

This is intended to make it easier for developers to build on Mac OS X, without having to pull in all of XCode. You can keep using CMake if you prefer.

To test, use make -C src MACOSX=1 NONX86=1 SDL=1 NOASM=1 for a release build.

Left to do:

• Add a content bundling script to be run after building, and a flag to trigger doing that. MACOSX_BUNDLE maybe?
• Somehow get access to a Mac running PowerPC and figure out how to build a multi-platform binary.
• Add the proper magic to compile using gcc if requested. (Right now, compilation is done via LLVM/Clang)