Windows build dependencies

[John Ralls]

I’ve uploaded new pacman packages for libsoup and webkitgtk3 to sourceforge so that GnuCash builds with (mostly) the latest Mingw32. I’ve also added a new section, https://wiki.gnucash.org/wiki/Building_on_Windows#Dependencies, to the wiki with details of the adjustments needed to manually update an existing environment.

Setup_mingw64 sets up a new environment correctly.

Regards,
John Ralls