[winrt support] Adding macro to help check and test what API of the Windows Runtime is being used.

This change adds a macro BOOST_WINDOWS_FAMILY that can be used by individual Boost libraries to determine which Windows APIs are available for use. I received some feedback from chriskohlhoff that BOOST_WINDOWS_RUNTIME is a better name. Before making any changes I'm interested in hearing what others think.

This is part of the work mentioned on the Boost dev mailing list:

http://boost.2283326.n4.nabble.com/winrt-support-Adding-support-for-Windows-8-store-phone-to-Boost-libraries-tc4661713.html

I also fixed up a failing test case boost_no_cxx11_hdr_regex.ipp.