Assume GFlags isn't present if FOLLY_HAVE_LIBGFLAGS isn't defined

Summary: Because, if it follows the same pattern as a normal config flag, it simply won't be defined if it isn't available.

Reviewed By: yfeldblum

Differential Revision: D3571862

fbshipit-source-id: 3322ceb0ce349de16b29310b66d1333aa04da516

folly/portability/GFlags.h

@@ -18,7 +18,7 @@
 
 #include <folly/portability/Config.h>
 
-#if defined(FOLLY_HAVE_LIBGFLAGS) && (FOLLY_HAVE_LIBGFLAGS == 0)
+#if !FOLLY_HAVE_LIBGFLAGS
 // glog/logging.h is dependent on this implementation detail
 // being defined otherwise it undefines all of this -_-....
 //