autotest compilation mode marker

[Eric Blake]

I noticed that autotest goes out of its way to provide the -*-compilation-*- 
marker at the top of individual testsuite logs, trying to make life easier for 
emacs.  However, it currently has a couple of flaws.

First, the filename is relative to the top-level log, not the individual log.  
So when I open testsuite.dir/1/testsuite.log, emacs can't find the file with 
the error.  I think _AT_CHECK should be using an absolute, not relative, path 
to $at_srcdir.

Second, the top-level file comes up in emacs Fundamental mode.  For compilation 
mode to be useful at the top-level, as well as the individual logs, we need to 
ensure that testsuite.log is given a prefix that emacs can recognize.

Finally, I wanted to update the m4 testsuite to take advantage of compilation 
mode: it uses an awk script to parse m4.texinfo and created generated.at.  When 
one of those tests fails, I would rather use the emacs compilation mode 
features to go to the original line in m4.texinfo than the generated line in 
generated.at.  I was able to do this by manually adding
      printf ("echo \"$at_srcdir/%s:%d:\"\n", FILENAME, NR)
to the awk script, but that feels hackish (I shouldn't be using the 
undocumented $at_srcdir).  Should we add a new AT_LINE(file, line, [column]) 
macro (perhaps with a better name), which outputs an appropriate compilation 
line error directive into the log?  If file is absolute, use it as-is; 
otherwise assume it is relative to the directory containing the .at file with 
the AT_CHECK; and output a compilation line directive that will work in spite 
of testsuite log being generated in a different directory than the .at file.

-- 
Eric Blake