make clean after mf changes

[Dan Eble]

The Contributor’s Guide says,

> Also, if you modify any font definitions in the mf/ directory then you must 
> run make clean and make before running make test-redo. This will recompile 
> everything, whether modified or not, and takes a lot longer. 

I assume the superficial explanation for requiring “make clean" here is that 
the makefile dependencies are (or were) incomplete.  If you have deeper 
knowledge, I’d appreciate being told anything that would shorten my 
investigation into it.  “That’s old and shouldn’t be required anymore” or 
“Fixing it is hopeless!” would also be helpful.

Thanks in advance,
— 
Dan