Did you start with a clean build directory and rerun ./configure?  What ./configure options did you use?  Make sure that an existing install isn't getting in the way during a build, particularly with --enable-shared.  With a current tip of the default branch, I've built all three standard OS X configurations (default unix (non-shared), --enable-shared, and --enable-framework) on OS X 10.8 and "make install" succeeds for all of them with sysconfig.get_config_var("SHLIB_SUFFIX") returning ".so" as expecting.