Building documentation in parallel

[John W. Eaton]

On 29-Aug-2012, Rik wrote:

| 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.

Previously, I used different directories for texi2dvi and texi2pdf.
Does that matter?  Is there a possibility for collision if the cache
directories are the same for both and they happen to run at the same
time?

jwe