Set or clear Makefile variables HAVE_CLOCK_GETTIME and
HAVE_CLOCK_MONOTONIC based upon results of the checks (overriding
default values from config.mak.uname).
CLOCK_MONOTONIC isn't available on RHEL3, but there are still RHEL3
systems being used in production.
Signed-off-by: Reuben Hawkins <reubenhwk@gmail.com> Reviewed-by: Eric Sunshine <sunshine@sunshineco.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>