Some clang inspired cleanups and update to latest autoconf

I was trying to build under mingw64 with clang, and got a lot of complaints and warnings. SO I

• Cleaned them up
• Removed post.ui.pgp.h because all that's necessary is to have a #ifdef in post.ui.h, which means you don't need to make the same changes in 2 places
• Cleaned up issues reported by -fsanitise=undefined
• Ran autopdate and cleaned up all the warnings I was getting from autoconf
• Removed -mwin32 switch from windows compile. It isn't necessary (and isn't recognised by clang!)
• made wininstall.py work correctly with different msys2 package sets

Note: I can't make a working version with msys2/clang64 - or at least it does work, but performs abysmally - but msys2/ucrt64 works fine and appears to perform a little better