scripts/sphinx-pre-install: add a script to check Sphinx install

Solving Sphinx dependencies can be painful. Add a script to
check if everything is ok.

Tested on:
	- Fedora 25 and 26;
	- Ubuntu 17.04;
	- OpenSuse Tumbleweed;
	- Arch Linux;
	- Gentoo.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>