genrst calls xsltproc to generate gui_flow.svg.

genrst calls xsltproc to generate gui_flow.svg.

Unfortunately, Sphinx doesn't seem to recognize that gui_flow.xml will
turn into gui_flow.svg after graph.xsl's transformation (but perhaps
I'm just doing something wrong?).  We work around that by running the
transformation and linking hacking.txt to the generated svg file
instead of the source xml.  Of course, this adds an additional build
dependency (xsltproc), but xslt processors should be common enough
that that's not a big deal.

The change was made in the scons cache file doc/.sconsrc-sphinx.
I also added the generated file (doc/img/gui_flow/gui_flow.svg) to
.hgignore.