maint: fix "make maintainer-clean" not working in tests/functional

Signed-off-by: Jan Pokorný <[email protected]>

tests/functional/GNUmakefile

@@ -3,11 +3,12 @@ all check:
 	@echo "Use 'make $@' within particular subdirectories"
 install: force
 	# definitely not desired to install anything from this subtree
-distclean:
-	# following is a nasty hack to keep "make distclean" succeeding
-	# (problem mostly arises from shared object files and hence shared
-	# compiler-generated makefile includes which are swiped when
-	# processing one subdir and missing as hard error for the other)
+distclean maintainer-clean:
+	# following is a nasty hack to keep "make distclean" (and
+	# ~ "maintainer-clean") succeeding (problem mostly arises from
+	# shared object files and hence shared compiler-generated makefile
+	# includes, which are swiped when processing one subdir and then
+	# missing, as a hard error, for the other)
 	@$(MAKE) -C log_external $@
 	@mkdir .deps
 	@touch .deps/log_client.Po .deps/log_interlib.Plo .deps/log_interlib_client.Po
@@ -17,4 +18,4 @@ distclean:
 	@$(MAKE) -f Makefile $@
 force: ;
 
-.PHONY: check distclean force install
+.PHONY: check distclean force install maintainer-clean