It can only be used for projects that don't have any rules at all, i.e.
they are purely using Meson to:
- configure files
- run (script?) tests
- install files that exist by the end of the setup stage
This can be useful e.g. for Meson itself, a pure python project.
When a subproject is disabled on the initial configuration we should not
add it into self.coredata.initialized_subprojects because that will
prevent calling self.coredata.init_builtins() on a reconfigure if the
subproject gets enabled.
Fixes: #10225.
Adds a new debug() function that can be used in the meson.build to
log messages to the meson-log.txt that will not be printed to stdout
when configuring the project.
This adds a new category of tests that does not need to run on all
platforms during CI. For now only run them on Linux runners because they
are not the bottleneck.