Add support for VS2015 (VC++ 14.0)

Add support for VS2015 (VC++ 14.0)

Due to the rewritten CRT in this version of Visual C++ it is no longer
possible (or at least not at all easy) to make use of the ioinfo struct,
which commit b47a847f62 (re-)introduced in order to fix RT#120091/118059.
Therefore, we effectively revert commit b47a847f62 for VS2015 onwards on
the basis that being able to build with VS2015 onwards is more important
than the RT#120091/118059 bug fix. This does unfortunately mean that perls
built with <=VS2013 will not be compatible with perls built with >=VS2015,
but they may well not have been compatible anyway because of the CRT
rewrite, and certainly wouldn't be compatible if perl builds with VS2015
were not supported!

See RT#125714 for more discussion about this.