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.
right, but if you're gonna fix it, fix it so there's no edge conditions lying around and 2 years down the road someone wonders why xlc works but everything else is broken...