makefiles: add TOCLEAN and TODISTCLEAN variables

So that includers of src/prog.mk or src/so.mk can just define anything
extra that needs to be cleaned without having to override the "clean"
target (or having to declare a "distclean" target).

Example usage:

    TOCLEAN += foo
    TODISTCLEAN += bar

Author: kmk3

src/prog.mk

@@ -22,7 +22,7 @@ $(PROG): $(OBJS) $(ROOT)/config.mk
 	$(CC)  $(LDFLAGS) -o $@ $(OBJS) $(LIBS) $(EXTRA_LDFLAGS)
 
 .PHONY: clean
-clean:; rm -fr *.o $(PROG) *.gcov *.gcda *.gcno *.plist
+clean:; rm -fr *.o $(PROG) *.gcov *.gcda *.gcno *.plist $(TOCLEAN)
 
 .PHONY: distclean
-distclean: clean
+distclean: clean; rm -fr $(TODISTCLEAN)

src/so.mk

@@ -19,7 +19,7 @@ $(SO): $(OBJS) $(ROOT)/config.mk
 	$(CC) $(LDFLAGS) -shared -fPIC -z relro -o $@ $(OBJS) -ldl
 
 .PHONY: clean
-clean:; rm -fr $(OBJS) $(SO) *.plist
+clean:; rm -fr $(OBJS) $(SO) *.plist $(TOCLEAN)
 
 .PHONY: distclean
-distclean: clean
+distclean: clean; rm -fr $(TODISTCLEAN)