[boost] [Config] N3276 decltype support detection missed for VC++12 (Ticket #9410)

As you know, VC++12 (aka 2013) completed N3276 decltype support. (See, e.g. http://msdn.microsoft.com/en-us/library/vstudio/hh567368.aspx http://thread.gmane.org/gmane.comp.lib.boost.devel/233752/focus=239118 ) Unfortunately the detection code for VC++12's N3276 decltype support hasn't been updated yet, as reported in #9410 (decltype N3276 for MSVC12 + compile fix for tests) https://svn.boost.org/trac/boost/ticket/9410 Ticket #9410 has a patch for this issue. The patch also fixes the test code of Boost.Config. Regards, Michel