Merge branch 'ab/make-clean-depend-dirs'

"make clean" has been updated to remove leftover .depend/
directories, even when it is not told to use them to compute header
dependencies.

* ab/make-clean-depend-dirs:
  Makefile: clean .depend dirs under COMPUTE_HEADER_DEPENDENCIES != yes

Author: gitster

Makefile

@@ -2459,7 +2459,6 @@ dep_args = -MF $(dep_file) -MQ $@ -MMD -MP
 endif
 
 ifneq ($(COMPUTE_HEADER_DEPENDENCIES),yes)
-dep_dirs =
 missing_dep_dirs =
 dep_args =
 endif