Re: Remove support for Visual Studio 2013

Maybe consider removing this workaround?  The original problem report indicated
that it didn't affect later versions:

src/backend/optimizer/path/costsize.c:  /* This apparently-useless variable dodges a compiler bug in VS2013: */

I'm not sure if it's worth removing this one, though:

src/port/strtof.c: * On Windows, there's a slightly different problem: VS2013 has a strtof()