build: always attempt to update util/build_version.cc
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

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