[patch] Define Feature-test macro for std::bool_constant

[Jonathan Wakely]

The latest feature-testing document defines the
__cpp_lib_bool_constant macro, so this uncomments the placeholder I
left in <type_traits>.

Tested powerpc64le-linux, committed to trunk.



commit 00ce7154a868daa8195d5bbc0b04370edf3e0de8
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Wed Oct 21 07:34:28 2015 +0100

    	* include/std/type_traits (__cpp_lib_bool_constant): Define.

diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits
index 55ca916..e291047 100644
--- a/libstdc++-v3/include/std/type_traits
+++ b/libstdc++-v3/include/std/type_traits
@@ -93,7 +93,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     using __bool_constant = integral_constant<bool, __v>;
 
 #if __cplusplus > 201402L
-// TODO: #define __cpp_lib_bool_constant 201505
+# define __cpp_lib_bool_constant 201505
   template<bool __v>
     using bool_constant = integral_constant<bool, __v>;
 #endif