FORCE_GIT_SHA

Summary: In third-party2 build we need to force git sha because we're compiling from a different git repositry.

Test Plan: `FORCE_GIT_SHA=igor make`

Reviewers: kradhakrishnan, sdong

Reviewed By: kradhakrishnan

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D38679

Makefile

@@ -177,7 +177,11 @@ CXXFLAGS += $(WARNING_FLAGS) -I. -I./include $(PLATFORM_CXXFLAGS) $(OPT) -Woverl
 LDFLAGS += $(PLATFORM_LDFLAGS)
 
 date := $(shell date +%F)
+ifdef FORCE_GIT_SHA
+	git_sha := $(FORCE_GIT_SHA)
+else
 	git_sha := $(shell git rev-parse HEAD 2>/dev/null)
+endif
 gen_build_version =							\
   printf '%s\n'								\
     '\#include "build_version.h"'					\