CI and downstream packagers have been using the Meson build for a while
now, and we checked that it's idempotent to the Autotools build.
Having two build systems in tree doesn't make maintaining and releasing
GTK any easier, even if it's the stable/frozen branch.
I couldn't get all reftests to work reliably, so the tests failing
on either CI or on my machine are skipped for now.
Installed tests are disabled by default and can be enabled with "-Dinstalled_tests=true"
This is not much of a test, tbh. But it will give us a convenient
dump of all GTK+ settings in continuous, which helps with tracking
down test failures that might depend on settings.
Makefile.decl does not work well with scripts that stay
in srcdir, so generate test-simplify from test-simplify.in,
just os that it ends up in builddir.
