Detect relaxed constexpr rules for gcc versions

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@250802 91177308-0d34-0410-b5e6-96231b3b80d8

include/__config

@@ -451,8 +451,11 @@ namespace std {
 #define _LIBCPP_HAS_NO_CONSTEXPR
 #endif
 
-// No version of GCC supports relaxed constexpr rules
+// Determine if GCC supports relaxed constexpr
+#if !defined(__cpp_constexpr) || __cpp_constexpr < 201304L
 #define _LIBCPP_HAS_NO_CXX14_CONSTEXPR
+#endif
+
 // GCC 5 will support variable templates
 #define _LIBCPP_HAS_NO_VARIABLE_TEMPLATES