core.stdc.math: proper support for Microsoft C Runtime (Visual Studio >= 2013)

These commits prepare <tt>core.stdc.math</tt> for full support of Microsoft's upcoming runtime with improved C99 support. The annoying differences between 32 and 64 bit platforms are taken care of.
It'll also be relevant for LDC.
Based on the C headers from VS 2014 CTP 3.
I wouldn't generally restrict the MSVCRT support to the last version either, but 2014 (and already 2013 for the C99 math stuff) represents a big leap in terms of portability and allows to simplify druntime and Phobos.
I've added comments about the first supported version (in this case, 2013). Having to support older versions of MSVCRT would imply additional ```version()``` nightmare which I'm strictly against.