build: always attempt to update util/build_version.cc

Summary:
This fixes two bugs: "make clean" would never remove the generated
file, util/build_version.cc, and since D33591, would be regenerated
only if it were absent.
* Makefile (clean): Remove the generated file.
(util/build_version.cc): Depend on the no-prereq FORCE target,
so that this target's rules are always run.
Since this is a generated file, make it read-only.
Also, be sure to remove the temporary file when it is the same
as the original.

Test Plan:
Ensure that we attempt regeneration every time.
Make it empty with an up-to-date time stamp and demonstrate
that it is rebuilt with the expected content:

  $ : > util/build_version.cc
  $ make util/build_version.o
   GEN      util/build_version.cc
   GEN      util/build_version.d
   GEN      util/build_version.cc
   CC       util/build_version.o
  $ cat util/build_version.cc
  #include "build_version.h"
  const char* rocksdb_build_git_sha = "rocksdb_build_git_sha:v3.10-2-gb30e72a";
  const char* rocksdb_build_git_date = "rocksdb_build_git_date:2015-03-27";
  const char* rocksdb_build_compile_date = __DATE__;

Reviewers: igor.sugak, sdong, ljin, igor, rven

Reviewed By: rven

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D36087

Makefile

@@ -163,10 +163,13 @@ gen_build_version =							\
 # as a regular source file as part of the compilation process.
 # One can run "strings executable_filename | grep _build_" to find
 # the version of the source that we used to build the executable file.
-util/build_version.cc:
-	$(AM_V_GEN)$(gen_build_version) > $@.tmp
-	$(AM_V_at)if test -f $@; then \
-	  cmp -s $@.tmp $@ && : || mv -f $@.tmp $@; else mv -f $@.tmp $@; fi
+FORCE:
+util/build_version.cc: FORCE
+	$(AM_V_GEN)rm -f $@-t
+	$(AM_V_at)$(gen_build_version) > $@-t
+	$(AM_V_at)if test -f $@; then					\
+	  cmp -s $@-t $@ && rm -f $@-t || mv -f $@-t $@;		\
+	else mv -f $@-t $@; fi
 
 LIBOBJECTS = $(LIB_SOURCES:.cc=.o)
 MOCKOBJECTS = $(MOCK_SOURCES:.cc=.o)
@@ -396,6 +399,7 @@ unity: unity.o
 
 clean:
 	rm -f $(PROGRAMS) $(TESTS) $(LIBRARY) $(SHARED) make_config.mk unity.cc
+	rm -f util/build_version.cc
 	rm -rf ios-x86 ios-arm scan_build_report
 	find . -name "*.[oda]" -exec rm {} \;
 	find . -type f -regex ".*\.\(\(gcda\)\|\(gcno\)\)" -exec rm {} \;