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.
on windows, the configure check that some dll/exe are present.
these are in fact automatically downloaded by the ./download script.
but that script is run _after_ ./configure.
So the tests are postponed and run in a post_download script that is
invoked at the end of the download script.