Merge branch 'rw/make-needs-librt'

Makefile assumed that -lrt is always available on platforms that want to use clock_gettime() and CLOCK_MONOTONIC, which is not a case for recent Mac OS X. The necessary symbols are often found in libc on many modern systems and having -lrt on the command line, as long as the library exists, had no effect, but when the platform removes librt.a that is a different matter--having -lrt will break the linkage. This change could be seen as a regression for those who do need to specify -lrt, as they now specifically ask for NEEDS_LIBRT when building. Hopefully they are in the minority these days. * rw/make-needs-librt: config.mak.uname: define NEEDS_LIBRT under Linux, for now Makefile: add NEEDS_LIBRT to optionally link with librt