included file names in error msgs

[Akim Demaille]

src/ace/doc % LANGUAGE=C make                                    nostromo Err 2
cd . \
  && /bin/sh /home/lrde/prof/akim/src/ace/config/missing --run makeinfo 
--no-split `echo autoconf.texi | sed 's,.*/,,'`
autoconf.texi:11755: Node `Environment Variables' previously defined at line 
201.

actually it is defined in install.texi:201, not autoconf.texi as the
msg seem to say.  How about mocking gcc?

/tmp % head foo.c bar.c                                          nostromo Err 1
==> foo.c <==
#include "bar.c"
typedef int foo;

==> bar.c <==
typedef int foo;
/tmp % gcc -c foo.c                                              nostromo 14:28
foo.c:2: redefinition of `foo'
bar.c:1: `foo' previously declared here


This is nice with Compilation-mode, to be able to click on either location.