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.
I tried that originally (as you can see by the file I accidentally left in this branch). The issue is that mkmf does a funky thing where it includes MakeMakeFile module in itself, which makes monkeypatching NOT work at all.