D lang compilers have an option -release (or similar) which turns off
asserts, contracts, and other runtime type checking. This patch wires
that up to the b_ndebug flag.
Fixes#7082
Sometimes it is convenient to run an arbitrary command (e.g. 'git diff')
on all subprojects.
Add a 'meson subprojects foreach' command to take care of that.
For this command the common argument 'subprojects' does not make sense,
so only add '--sourcedir' and cover the case of a missing
options.subprojects in run().
This allows us to more easily have the documentation in sync with
the source code as people will have to document new features etc
right at the time where they implement it.