cut outdated use of FOLLY_USE_LIBSTDCPP

Summary: Just replace it with `__GLIBCXX__`.

Reviewed By: andrewjcg

Differential Revision: D32624136

fbshipit-source-id: fa3ae4b4e7c3b6c05793a25cc99a93a0d0c18c7f

folly/Portability.h

@@ -483,7 +483,7 @@ constexpr auto kIsLibcpp = true;
 constexpr auto kIsLibcpp = false;
 #endif
 
-#if FOLLY_USE_LIBSTDCPP
+#if __GLIBCXX__
 constexpr auto kIsLibstdcpp = true;
 #else
 constexpr auto kIsLibstdcpp = false;