AC_SUBST also EXEEXT_FOR_BUILD and use that in Makefile.in.

As winemv.set.sh is now called WindowsMSVCEnv.Set.sh, with capital E
and S like all the others, we can simplify the glob pattern for the
Set.sh file.

Don't attempt to download and/or run unpackers for dependencies
relevant only when using MSVC if using MinGW.

Misc other Windows host vs. build fixes.