Correct VS2015's impending CCTYPE

Correct VS2015's impending CCTYPE

VS2013 was version 12.0, so VS2015 would be expected to be 13.0, not 15.0,
so the CCTYPE would be MSVC130, not MSVC150. However, VS2015 (just released
on 20 July) actually installs to somewhere like "C:\Program Files (x86)\
Microsoft Visual Studio 14.0" and sets "VisualStudioVersion=14.0" in the
environment, so they seem to have skipped version 13.0 (superstition?!) and
gone to version 14.0 instead. So our next CCTYPE will be MSVC140.

Unfortunately, it currently doesn't work. Changes to the CRT give linker
problems that are fixed by dropping -nodefaultlib, but changes to the FILE
structure are more of a problem. Tweaks in Makefile and config_sh.PL to
undef d_stdiobase/USE_STDIO_BASE and d_sdtsdtio/USE_STDIO_PTR etc go some
way to help, but there is still trouble compiling perlio.c (which accesses
f->_file in PerlIOStdio_invalidate_fileno) and win32/win32.c (which
accesses (pf)->_flag in win32_fdupopen).

(This commit also changes referencs to version N.x to simply N.0 since they
all are .0 only, except for the .NET 2002/2003 releases, which were 7.0/7.1
respectively.)