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>

Documentation/Makefile

@@ -28,6 +28,8 @@ ifeq ($(HAVE_SPHINX),0)
 
 else # HAVE_SPHINX
 
+export SPHINXOPTS = $(shell perl -e 'open IN,"sphinx-build --version 2>&1 |"; while (<IN>) { if (m/([\d\.]+)/) { print "-jauto" if ($$1 >= "1.7") } ;} close IN')
+
 # User-friendly check for pdflatex and latexmk
 HAVE_PDFLATEX := $(shell if which $(PDFLATEX) >/dev/null 2>&1; then echo 1; else echo 0; fi)
 HAVE_LATEXMK := $(shell if which latexmk >/dev/null 2>&1; then echo 1; else echo 0; fi)