Simplify build-time config checks.

These now simply reflect whatever config.hpp reports, and are therefore always in synch with the headers.
They are also now compile-time tests only, which means they function with cross compilers.
Fixes: https://svn.boost.org/trac/boost/ticket/12805.

test/config_test.cpp

@@ -1,4 +1,4 @@
-//  This file was automatically generated on Sun Feb  5 19:09:22 2017
+//  This file was automatically generated on Mon Apr 17 12:45:44 2017
 //  by libs/config/tools/generate.cpp
 //  Copyright John Maddock 2002-4.
 //  Use, modification and distribution are subject to the