Import patch from libc++ r197313 which allows using libc++ headers with gcc

Differential Revision:	https://reviews.freebsd.org/D942
Reviewed by:	imp

contrib/libc++/include/__config

@@ -374,6 +374,7 @@ namespace std {
 #define _LIBCPP_HAS_NO_VARIADICS
 #define _LIBCPP_HAS_NO_RVALUE_REFERENCES
 #define _LIBCPP_HAS_NO_ALWAYS_INLINE_VARIADICS
+#define _LIBCPP_HAS_NO_STRONG_ENUMS
 
 #else  // __GXX_EXPERIMENTAL_CXX0X__