makefile: refactor "clean" target; add "distclean"

* clean now just removes the object files, dwo debugging objects, .cmake
  and intermediary .bin files but keeps CMakeCache and similar
* a new target - distclean - is introduced to do what was previously
  done by "clean", essentially: completely remove the build folders

Makefile

@@ -14,7 +14,7 @@ v_env = VALGRIND=
 v_db = $(d_folder)_valgr
 v_rl = $(r_folder)_valgr
 
-.PHONY: check tidy build debug release valgrind san test clean
+.PHONY: check tidy build debug release valgrind san test clean distclean
 
 debug:
 	if [ ! -d "$(d_folder)" ]; then mkdir -pv $(d_folder); fi
@@ -62,5 +62,8 @@ build: debug
 test: check build
 
 clean:
-	find . -iwholename '*cmake*' -not -name CMakeLists.txt -delete
+	@find . -regextype posix-egrep -regex ".*\.(o|dwo|out|bin|cmake)" -delete
+
+distclean:
+	@find . -iwholename '*cmake*' -not -name CMakeLists.txt -delete