revise gflags portability namespaces

Summary: Let `folly::gflags` be an alias to the true gflags namespace if gflags is available.

Reviewed By: Gownta

Differential Revision: D65899780

fbshipit-source-id: 7e050d5268f484f92b58d179005c4ce79f3cee52

openr/kvstore/tests/DualTest.cpp

@@ -1020,7 +1020,7 @@ int
 main(int argc, char* argv[]) {
   // Parse command line flags
   testing::InitGoogleTest(&argc, argv);
-  gflags::ParseCommandLineFlags(&argc, &argv, true);
+  folly::gflags::ParseCommandLineFlags(&argc, &argv, true);
   google::InitGoogleLogging(argv[0]);
   google::InstallFailureSignalHandler();
   FLAGS_logtostderr = true;