[PATCH] Makefile: add NO_CLOCK_GETTIME check

[Reuben Hawkins <reubenhwk@gmail.com>]

Some really old platforms, rhel3, which don't define CLOCK_MONOTONIC, still
need a way to build git.  Some high resolution timer code was added Jul 12,
2014 to aid debugging performance issues.  That code uses clock_gettime and
CLOCK_MONOTONIC.  That change breaks building git on ancient platforms which
don't support CLOCK_MONOTONIC, so a way to disable that code is needed.

To use this feature...

$ make NO_CLOCK_GETTIME=1

This change amends the commit...

 commit 148d6771bf5e00aa1d7fa2221507a3dfe4c1e37f
 Author: Karsten Blees <karsten.blees@gmail.com>
 Date:   Sat Jul 12 02:05:42 2014 +0200

  trace: add high resolution timer function to debug performance issues
---
 Makefile | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/Makefile b/Makefile
index 9f984a9..0c1c8cf 100644
--- a/Makefile
+++ b/Makefile
@@ -1502,10 +1502,12 @@ ifdef GMTIME_UNRELIABLE_ERRORS
 	BASIC_CFLAGS += -DGMTIME_UNRELIABLE_ERRORS
 endif
 
+ifndef NO_CLOCK_GETTIME
 ifdef HAVE_CLOCK_GETTIME
 	BASIC_CFLAGS += -DHAVE_CLOCK_GETTIME
 	EXTLIBS += -lrt
 endif
+endif
 
 ifeq ($(TCLTK_PATH),)
 NO_TCLTK = NoThanks
-- 
1.9.1