New formula: gccxml

GCC-XML was removed from homebrew (#12764) some time ago (No stable download, failed to build with clang (#11043)).

GCC-XML can now be found on github (https://github.com/gccxml/gccxml), with some updates to make it work with the latest OS X versions (I tested the build on OS X 10.7 and 10.8). The last tagged version is 0.6 on github, so for the moment this formula is head only.