[RFA] script to update our gnulib-import (take 2)

[Joel Brobecker]

> > Joel>         * gnulib/update-gnulib.sh: New script.
> > 
> > I think it is fine.
> 
> Thanks, Tom. Applied.

I've also looked at our internals manual as well as wiki. gnulib
wasn't referenced anywhere, but there was a "Developer Tips" page
which had instructions on how to regenerate the configure after
changing gdb/configure.ac. So I added a new section there about
gnulib as well.

-- 
Joel