Add new build-time configuration feature.

test/config_build_check.cpp

@@ -0,0 +1,18 @@
+//  Copyright John Maddock 2014.
+//  Use, modification and distribution are subject to the 
+//  Boost Software License, Version 1.0. (See accompanying file 
+//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+//  See http://www.boost.org/libs/config for the most recent version.//
+//  Revision $Id$
+//
+
+#  include "../test/boost_has_int128.ipp"
+#  include "../test/boost_no_constexpr.ipp"
+#  include "../test/boost_no_cxx11_user_lit.ipp"
+
+int main( int, char *[] )
+{
+   return boost_has_int128::test() || boost_no_cxx11_constexpr::test() || boost_no_cxx11_user_defined_literals::test();
+}
+