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.
If LIBPATH is fucking the rpath, then unset the LIBPATH entirely.  Make ruby stop trying to be so stupidly helpful when the compiler flags already know how to wire up the rpath on binaries right.  The xlc binary itself has an rpath that finds the right stuff in /usr/lib, so setting LIBPATH to anything just gets that wrong.  Similarly, when we compile ruby and gems we use the right `-Wl,-rpath` options so that the embedded path is wired up in the binary.  Then when I invoke things on the command line I don't have to set LIBPATH and everything just works.  Ruby is going out of its way to wire up LIBPATH and break all the system utils, which seem to be the real root cause of this problem.  It is unnecessary and there is no single correct LIBPATH for all binaries.