Merge branch 'rw/make-needs-librt'

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