Makefile: add NEEDS_LIBRT to optionally link with librt

We unconditionally link with librt, when HAVE_CLOCK_GETTIME is defined.
But clock_gettime() has been available in most libc implementations for
some time now (e.g., for glibc since version 2.17) and no longer
requires linking with librt. Furthermore, commit a6c3c63 (configure.ac:
check for clock_gettime() and CLOCK_MONOTONIC) will automatically
determined which library (libc or librt) is required for linking when
checking for clock_gettime().

The assumption to unconditionally link with librt was OK, since either
almost every Unix-like system provides a version of librt for backwards
compatibility or other systems, namely Windows or OS X, never provided
clock_gettime(). However, in the latest release of OS X (macOS Sierra),
this function has been added to OS X libc version. As a result, when
running the configuration script, HAVE_CLOCK_GETTIME is set and since
librt is not present, it causes a linker error.

This patches requires those not building via the configuration scripts
to define NEEDS_LIBRT in addition to HAVE_CLOCK_GETTIME, if needed.

Signed-off-by: Ronald Wampler <rdwampler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -351,9 +351,12 @@ all::
 # Define GMTIME_UNRELIABLE_ERRORS if your gmtime() function does not
 # return NULL when it receives a bogus time_t.
 #
-# Define HAVE_CLOCK_GETTIME if your platform has clock_gettime in librt.
+# Define HAVE_CLOCK_GETTIME if your platform has clock_gettime.
 #
-# Define HAVE_CLOCK_MONOTONIC if your platform has CLOCK_MONOTONIC in librt.
+# Define HAVE_CLOCK_MONOTONIC if your platform has CLOCK_MONOTONIC.
+#
+# Define NEEDS_LIBRT if your platform requires linking with librt (glibc version
+# before 2.17) for clock_gettime and CLOCK_MONOTONIC.
 #
 # Define USE_PARENS_AROUND_GETTEXT_N to "yes" if your compiler happily
 # compiles the following initialization:
@@ -1465,13 +1468,16 @@ endif
 
 ifdef HAVE_CLOCK_GETTIME
 	BASIC_CFLAGS += -DHAVE_CLOCK_GETTIME
-	EXTLIBS += -lrt
 endif
 
 ifdef HAVE_CLOCK_MONOTONIC
 	BASIC_CFLAGS += -DHAVE_CLOCK_MONOTONIC
 endif
 
+ifdef NEEDS_LIBRT
+	EXTLIBS += -lrt
+endif
+
 ifdef HAVE_BSD_SYSCTL
 	BASIC_CFLAGS += -DHAVE_BSD_SYSCTL
 endif