Use the correct gsubrc extension on OS X

I think I'm supposed to use LT_MODULE_EXT or LTDL_SHLIB_EXT though...

However:
> Yes, well, .so is the right extension on Mac OS X for loadable modules.
> .dylib is the right extension for shared libraries.
-- <http://lists.gnu.org/archive/html/autoconf/2009-09/msg00045.html>

Based-on: nandub/gale-old@50172814e3c347ce400455f691ecbfbffad9f25c