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().
* If clone-recursive is specified, populate submodules only after
the specified revision/branch has been checked out. Fixes#4996
* Update submodules recursively on `meson subprojects update`
Daniel Mensinger