Improvements for maint.mk

[Martin von Gagern]

Hello!

As I just wrote a lengthy email of issues I noticed with maint.mk, here are
a few patches dealing with some of them.  I hope you can apply them to
gnulib.

As for the copyright: I've already assigned copyright for wdiff to the fsf.
Do I have to repeat all the paperwork, or can I simply commit my
modifications to the wdiff tree in order to make them officially owned by
the FSF?

Greetings,
 Martin von Gagern