VPATH elimination by configure

[Akim Demaille]

It seems to me that Autoconf's job is not to remove unportable code
written by the user, so it seems wrong to have it remove VPATH
definitions from Makefiles.  Historically I'm sure it was useful, in a
previous century, in a previous millennium.  Today, I tend to think
automake should *warn* when it sees VPATH in a Makefile.am, and
autoconf should shut up and do nothing.

Comments?

(This, of course, is totally unrelated with the possible existence of
tests for VPATH support, which is fully under Autoconf's natural
responsibility).