core.stdc.stdio: prepare for MSVCRT 14 (VS 2015) support

[kinke]

Once MSVC 2014 is required, things will get easier, for example here because they now provide a standard-conformant (v)snprintf() implementation (except for x87 support), see this MSDN blog.
Additionally, _fputc_nolock() and _fgetc_nolock() are no longer defined as macros, but available in the library, just like any other supported C runtime.

[kinke]

Btw this really ought to be checked for the other platforms too. A 32-bit int is highly unlikely.

[Orvid]

This should be using a separate version statement for the 2014 version. There is no reason that earlier versions should be prevented from building DMD and D code.

This change also probably breaks all code that called snprintf using a string modifier.

Lastly, don't half indent version statements.

[kinke]

The 2014 version (well, so much time has passed for such a mini-PR, that the VS version has been advanced to 2015 in the meantime :o)) is disabled and only there to urge the responsible people to move on with Windows and MSVCRT support.
Once enabled, it may break code iff that code was written explicitly for MSVCRT, i.e., using the non-standard old modifiers. And imo that's highly unlikely, given the current MSVCRT support. In fact, it should fix portability issues. :]

[kinke]

The last commit is by no means complete.
What would be needed is something very similar to kinke@3c897cf.
Given that there's absoletely no progress regarding my DMD PRs, I'll focus on LDC only in the future, where contribution is still appreciated.