config.mak.uname: define NEEDS_LIBRT under Linux, for now

My Debian wheezy LTS system is still on glibc 2.13; and LTS
distros may use older glibc, still, so lets not unnecessarily
break things out-of-the-box.

We seem to assume Linux is using glibc in our Makefiles anyways,
so I don't think this will introduce new breakage for users of
alternative libc implementations.

Signed-off-by: Eric Wong <e@80x24.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

config.mak.uname

@@ -36,6 +36,8 @@ ifeq ($(uname_S),Linux)
 	HAVE_DEV_TTY = YesPlease
 	HAVE_CLOCK_GETTIME = YesPlease
 	HAVE_CLOCK_MONOTONIC = YesPlease
+	# -lrt is needed for clock_gettime on glibc <= 2.16
+	NEEDS_LIBRT = YesPlease
 	HAVE_GETDELIM = YesPlease
 	SANE_TEXT_GREP=-a
 endif