Enable __is_trivially* intrinsics for GCC 5.1

Until GCC 5.1 the __is_trivially* intrinsics were not provided. Enable use of
the builtins for GCC 5.1.

Also enable Reference qualified member functions for GCC 4.9 and greater.

This patch also defines _GNUC_VER to 0 when __GNUC__ is not defined because
libc++ assumes _GNUC_VER is always defined.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@239653 91177308-0d34-0410-b5e6-96231b3b80d8

include/__config

@@ -17,6 +17,8 @@
 
 #ifdef __GNUC__
 #define _GNUC_VER (__GNUC__ * 100 + __GNUC_MINOR__)
+#else
+#define _GNUC_VER 0
 #endif
 
 #if !_WIN32