Use more libstdc++ specific macro

Summary:
- The __GNUC__ macro is used by other compilers like Clang and
  ICC, but ext/stdio_filebuf.h is a libstdc++ only extension.

fixes #636
Closes https://github.com/facebook/folly/pull/637

Reviewed By: yfeldblum

Differential Revision: D5434532

Pulled By: Orvid

fbshipit-source-id: ce4f966a74aa9f82ae907c67807ac87ec1588173

folly/experimental/symbolizer/Symbolizer.cpp

@@ -23,7 +23,7 @@
 #include <cstdlib>
 #include <iostream>
 
-#ifdef __GNUC__
+#ifdef __GLIBCXX__
 #include <ext/stdio_filebuf.h>
 #include <ext/stdio_sync_filebuf.h>
 #endif