With 3.4, the tip of the 'latest' branch will be a merge commit for
3.3-stable and the main branch, and not simply a fast-foward to an
already tested commit.
This sets up a tiny additional safety net before that merge.
GitHub runners unfortunately do not come with MinGW or VS 2010
pre-installed, so moving those builds will be more involved. MinGW-w64
is not a good replacement for MinGW as it is far more complete.
This gives at least some feedback for all supported platforms via the
GitHub Actions system.
VS 2010 and MinGW are the two trickiest Windows environments so this
should hopefully catch more mistakes. This is probably not the right
way to set up any of this, but it works for now.
This is also a first step towards building our release binaries with CI.
[skip travis]
