Add options to disable gflags to folly::InitOptions

Summary:
As title.  CLI applications that don't use gflags might not care about
process-wide gflags, so add an option to kill it.

Reviewed By: yfeldblum

Differential Revision: D27066900

fbshipit-source-id: d412391879cd5ac89eba5903fd033775cf0ca49a

folly/init/Init.cpp

@@ -65,7 +65,9 @@ void init(int* argc, char*** argv, InitOptions options) {
 #if !FOLLY_HAVE_LIBGFLAGS
   (void)options;
 #else
+  if (options.use_gflags) {
     gflags::ParseCommandLineFlags(argc, argv, options.remove_flags);
+  }
 #endif
 
   folly::initLoggingOrDie(FLAGS_logging);

folly/init/Init.h

@@ -25,6 +25,7 @@ class InitOptions {
   InitOptions() noexcept;
 
   bool remove_flags{true};
+  bool use_gflags{true};
 
   // mask of all fatal (default handler of terminating the process) signals for
   // which `init()` will install handler that print stack traces and invokes
@@ -43,6 +44,11 @@ class InitOptions {
     fatal_signals = val;
     return *this;
   }
+
+  InitOptions& useGFlags(bool useGFlags) {
+    use_gflags = useGFlags;
+    return *this;
+  }
 };
 
 /*