Fix make clean

Commit 82399a9239 assumes you can override `make clean` on the fact
that prerequisites of the same target are merged. This is true but
unfortunately defining a `clean` target prevents automake from
generating its own one, hence the default clean actions are not
performed. This can be easily seen by inspecting the generated
`Makefile`.

Solve this issue by manually chaining up the default action.

Makefile.kbuild

@@ -13,15 +13,14 @@ modules_install:
 	$(KBUILD) modules_install
 
 
-# Call Kbuild clean *before* automake `clean` (see #5 for the rationale)
+# Override default `clean` target to call Kbuild clean *before* automake
-# by adding `kbuild-clean` to the clean prerequisites. Having targets
+# `clean` (see #5 for the rationale).
-# with the same name will result in merging the prerequisites, i.e.:
-# ```
-# # Equivalent to 'clean: prerequisite1 prerequisite2'
-# clean: prerequisite1
-# clean: prerequisite2
-# ```
 clean: kbuild-clean
+	@if test -z "$(SUBDIRS)"; then \
+	  $(MAKE) $(AM_MAKEFLAGS) clean-am ; \
+	else \
+	  $(MAKE) $(AM_MAKEFLAGS) clean-recursive ; \
+	fi
 
 kbuild-clean:
 	$(KBUILD) clean