make target to avoid rebuilding fonts

[Carl D. Sorensen]

I've been working on tracking down the loss of functionality for
Forbid_line_break_engraver.

To do this I have to check out various commits and see if they have the
functionality.

>From time to time I need to blast out the object files because make gives me
an error.

So I've been doing "make clean".  But this blasts away all the fonts, so I
have to do the rebuild on the fonts.

Can I either:

1) Use a make target that will clean everything but the fonts?

or
2) Copy the font tree somewhere, do make clean, then copy the font tree back
before I do make?

Thanks,

Carl