Update gflags to latest master

This means that gflags no longer leaks config.h, so I had to fix a bug
in glog.bzl where config.h is generated at the wrong path.

I also switched to the best-practice for depending on git repositories,
ie using http_archive with a mirror.

WORKSPACE

@@ -1,5 +1,9 @@
-git_repository(
+http_archive(
  name = "com_github_gflags_gflags",
- remote = "https://github.com/gflags/gflags.git",
- tag = 'v2.2.1',
+ sha256 = "6e16c8bc91b1310a44f3965e616383dbda48f83e8c1eaa2370a215057b00cabe",
+ strip_prefix = "gflags-77592648e3f3be87d6c7123eb81cbad75f9aef5a",
+ urls = [
+ "https://mirror.bazel.build/github.com/gflags/gflags/archive/77592648e3f3be87d6c7123eb81cbad75f9aef5a.tar.gz",
+ "https://github.com/gflags/gflags/archive/77592648e3f3be87d6c7123eb81cbad75f9aef5a.tar.gz",
+ ],
 )

bazel/glog.bzl

@@ -113,7 +113,7 @@ EOF
  'src/config.h.cmake.in',
  ],
  outs = [
- 'glog_internal/src/config.h',
+ 'glog_internal/config.h',
  ],
  cmd = "awk '{ gsub(/^#cmakedefine/, \"//cmakedefine\"); print; }' $< > $@",
  )