publish.tst failure when OSMesa excluded from build

[Daniel J Sebald]

There are two "make check" failures when --without-OSMesa option is used 
to build Octave.  These failure don't overly concern me, but I'm just 
pointing them out because it seems that they technically shouldn't be 
failures in the sense that Octave is able to figure out that gnuplot is 
available for use in its place during the documentation build stage.

Here are the messages:

  publish.tst ................................................. PASS 
   0/2
                                                                  FAIL    2

!!!!! test failed
__osmesa_print__: support for offscreen rendering with OSMesa was 
unavailable or disabled when Octave was built

One thing that could be done in this case is to modify the following 
test from publish.tst:

%!   if (ispc ())
%!     graphics_toolkit ("gnuplot");
%!   endif

to be something like

%! if (ispc () || NO_OSMESA_AVAILABLE())
%! graphics_toolkit ("gnuplot");
%! endif

That way there wouldn't be a "make check" error unless there is no 
OSMesa and no gnuplot available.

Is it worth altering?  How does Octave currently determine at runtime 
during the plot-figure build that no OSMesa is available?

Dan