docs: by default, build docs a lot faster with Sphinx >= 1.7

Since Sphinx version 1.7, it is possible to use "-jauto" in
order to speedup documentation builds. On older versions,
while -j was already supported, one would need to set the
number of threads manually.

So, if SPHINXOPTS is not provided, add -jauto, in order to
speed up the build. That makes it *a lot* times faster than
without -j.

If one really wants to slow things down, it can just use:

	make SPHINXOPTS=-j1 htmldocs

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
[ jc: fixed perl magic to determine sphinx version ]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>