Remove support for Visual Studio 2013

Hi all,

Cutting support for now-unsupported versions of Windows is in the air
for a couple of months, and while looking at the code a first cleanup
that looked rather obvious to me is the removal of support for VS
2013, as of something to do for v16~.

The website of Microsoft has only documentation for VS >= 2015 as far
as I can see.  Note also that VS can be downloaded down to 2012 on
their official website, and that the buildfarm members only use VS >=
2017.

The patch attached cleans up the following things proper to VS 2013:
- Locale handling.
- MIN_WINNT assignment.
- Some strtof() business, as of win32_port.h.
- Removal of _set_FMA3_enable() in main.c related to floating-point
operations.
- MSVC scripts, but that's less interesting considering the work done
with meson.

A nice result is that this completely removes all the checks related
to the version number of _MSC_VER from the core code, making the code
depend only on the definition if the flag.

Thanks,
--
Michael