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>
diff --git a/config.mak.uname b/config.mak.uname
index f3c93f2..b64b63c 100644
--- a/config.mak.uname
+++ b/config.mak.uname
@@ -35,6 +35,7 @@
 	LIBC_CONTAINS_LIBINTL = YesPlease
 	HAVE_DEV_TTY = YesPlease
 	HAVE_CLOCK_GETTIME = YesPlease
+	HAVE_CLOCK_MONOTONIC = YesPlease
 endif
 ifeq ($(uname_S),GNU/kFreeBSD)
 	HAVE_ALLOCA_H = YesPlease