Our official instructions only tell how to build with Meson, our CI is
only done with Meson and I wouldn't be surprised if most developers only
used Meson as well.
In other words, the autotools build is outdated. Let's say our goodbye
and drop it in the next stable release.
Allow parallel building and testing by using a single Makefile.am
Implement parallel testing using TAP, with various drivers and
compilers living in the build/ directory.
Fix all sorts of issues that this caused, including builddir != srcdir,
leaks in tests and so on.
It would have been nice to break out all the above into separate
commits ... blush.
