Update MinGW versions used for the binaries and provide links to the
working compilers.
Add "-rc" to some links to make them work.
Also update the binaries file names.
The actual version from https://github.com/wxWidgets/wxWidgets/releases/tag/v3.1.0
with the correct SHA1 sums, a few other minor edits and without the line
breaks which don't seem to be handled correctly in the release announcements.