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.
this is probably better done as a patch to ruby itself.  otherwise you have to track down any gem which might be installed into a ruby and make sure that all of them have a dep on ruby-customization if aix.   as it stands the ruby in omnibus-software is broken on aix unless you know about this hack which is kind of spooky-action-at-a-distance.