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

"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
diff --git a/Makefile b/Makefile
index b88b914..3df0ab5 100644
--- a/Makefile
+++ b/Makefile
@@ -2477,7 +2477,6 @@
 endif
 
 ifneq ($(COMPUTE_HEADER_DEPENDENCIES),yes)
-dep_dirs =
 missing_dep_dirs =
 dep_args =
 endif