windres is not always present whereas goversioninfo is easy to install.
This also fixes the generation of resource.syso to be part of the
standard build script instead of only the AppVeyor CI build.
Finally, adapt update-version.sh to patch versioninfo.json.
AppVeyor now has Go and Git for Windows 2.x installed by default. Also, we
can tell MSYS to actually create symbolic links. Finally, let the CI
create the installer, too.
This variable is a bit of a mystery to non-Go developers and it's
unclear how you set it up properly. As a result, if it's not set let's
set everything up how Go likes it. Ignore the `src` directory that
we're creating now.
This will also help keep the Homebrew formula simpler.
Closes#388.
