AIX mkmf fix for Ruby

On AIX when compiling with xlC, the library path should be set to /usr/lib first and foremost during gem installs. However, the compiler flags are set during the compilation of Ruby. This patch overrides the default libpath in the event that you are running a gem install with xlc as the compiler.
We don't have to care about gcc in this case, only xlc, since it's going to run xlc on all the things due to the rest of the compilation flags. You are correct tho, this is in no way 'complete'....