AIX mkmf fix for Ruby

:construction: 
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 wonder if this should be a dep of the `rubygems` software def. That would ensure gem installs on non-Chef AIX packages would also work.