Make building documantation parallelable.

sphinx will fail in a race condition, if every sphinx target
uses the same CACHE dir:

Exception occurred:
	File "/usr/lib/pymodules/python2.7/sphinx/cmdline.py", line 189, in main
app.build(force_all, filenames)
	File "/usr/lib/pymodules/python2.7/sphinx/application.py", line 204, in build
self.builder.build_update()
	File "/usr/lib/pymodules/python2.7/sphinx/builders/__init__.py", line 196, in build_update
	'out of date' % len(to_build))
	File "/usr/lib/pymodules/python2.7/sphinx/builders/__init__.py", line 238, in build
self.env.topickle(path.join(self.doctreedir, ENV_PICKLE_FILENAME))
	File "/usr/lib/pymodules/python2.7/sphinx/environment.py", line 351, in topickle
	movefile(filename + '.tmp', filename)
	File "/usr/lib/pymodules/python2.7/sphinx/util/osutil.py", line 104, in movefile
os.rename(source, dest)
	OSError: [Errno 2] No such file or directory