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

diff --cc Makefile
Simple merge