makefile: always cleanup build dir

Makefile

@@ -10,6 +10,7 b' OUTDATED_PACKAGES = ${PATH_TO_OUTDATED_P'
 clean:
 	make test-clean
 	find . -type f \( -iname '*.c' -o -iname '*.pyc' -o -iname '*.so' -o -iname '*.orig' \) -exec rm '{}' ';'
+	find . -type d -name "build" -prune -exec rm -rf '{}' ';'
 
 
 .PHONY: test