#2306 Add DIR_O to makefile

[Arkadiusz MIchalski]

Would be nice to have DIR_O in makefile, same as we see in scintilla.mak. Without it, I can't generate files by various tools without calling clean first. And this means that when I go back to the previous tool, I have to do the whole generation again.

This is simillar to lexilla issue: https://github.com/ScintillaOrg/lexilla/issues/50

I made the appropriate changes to the DepGen.py, deps.mak and makefile (added in the attachments).

[Neil Hodgson]

Committed as [121b40].