configure.ac: check for clock_gettime and CLOCK_MONOTONIC

configure.ac: check for clock_gettime and CLOCK_MONOTONIC

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>