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.
The only place to 'shim' that in is in mkmf, unfortunately, if we unset it across the board, miniruby won't be able to compile ruby.