Changes to build pan under windows

This adds changes to pan to enable building under mingw64/msys-2, Also added some instructions.

Fixes #143 (closed)

Note that wininstall.py only really works for gtk2. I haven't yet done gtk3 as it looks terrible (IMO) on windows and I will need to do some more work there.

Also this doesn't replicate the current windows installer, so there may be issues with language mappings

Also changed to use enchant-2 under gtk2, which works just as well, and is available under msys2/mingw64