Merge branch 'mk/make-rm-depdirs-could-be-empty'

"make COMPUTE_HEADER_DEPENDENCIES=no clean" would try to run "rm -rf $(dep_dirs)" with an empty dep_dir, but some implementations of "rm -rf" barf on an empty argument list. * mk/make-rm-depdirs-could-be-empty: Makefile: don't run "rm" without any files