Allow debugging internal commands

gnome.gtkdoc() is indirectly called through meson --internal gtkdoc. That means two things:

• We can't call ninja some_file_generated_by_gtkdoc-mkdb (it's called by meson --instal...)
• We can't see the "real" command (gtkdoc-mkdb) to check the passed arguments. That'd be great to have a --verbose parameter to pass to meson and meson --internal. Meson also should print the commands on fail (just as Ninja does). Another way to solve the problem would be to completely remove the --internal calls and let ninja do the job.