Use gflags with bazel the way it recommends

Also lets us remove the hand-generated headers which are now out of
date.

Change-Id: I8673300b6c8c4e7a32e68a07d48ac79cb59faa62
diff --git a/build_tests/BUILD b/build_tests/BUILD
index 12a979f..a341358 100644
--- a/build_tests/BUILD
+++ b/build_tests/BUILD
@@ -9,7 +9,7 @@
         "gflags.cc",
     ],
     deps = [
-        "//third_party/gflags",
+        "@com_github_gflags_gflags//:gflags",
     ],
 )