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.
so is that latter statement correct?  the stuff we compile and link against /opt/embedded/chef/lib should have an rpath which points at /opt/chef/embedded/lib.  we do a whole lot of work to make sure that is correct.

if LIBPATH is overriding whatever rpath is in the binary, then can we simply unset LIBPATH to get back to the binary rpath being correct again?