Add clean to makefile

[sergio-costas]

No description provided.

[robert-ancell]

I'm unsure what we want to do here - @jhenstridge do you have any opinions? Does this add any value over using git clean? This feels like the rules are leaving behind unnecessary artifacts (can be fixed), and there should only be a few files left that area easy to manually delete. If this is useful on CI then a git clean is probably going to be more reliable. If we need the build to be more reliable then perhaps it's time to stop using a simple Makefile.

[sergio-costas]

@robert-ancell The point is that git clean removes all the files that don't belong to the git repository; but I use a lot of custom scripts to avoid repeating things in the command line, or keep some .diff files around, so a make clean entry is useful to keep them.