Building documentation in parallel

[Rik]

8/29/12

All,

I checked in a changeset that I think solves the parallel build problem
completely (http://hg.savannah.gnu.org/hgweb/octave/rev/87f337783d95). 
There was a problem with the last attempt in that multiple directories were
being created.  Because the name of each directory had the process ID in
it, the directory name was unique and Tex could not find the cached
results.  Thus, every build was a new build which takes about 4X the time
of a cached build.  I renamed the directory for intermediate results to be
a fixed name (t2d_cache) and added a local clean rule to remove it if you
type 'make clean'.  I extended this change to the other doc directories as
well.

--Rik