scripts: sphinx-pre-install: address some issues with Gentoo

There are some small misdetections with Gentoo. While they
don't cause too much trouble, it keeps recomending to
install things that are already there.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/7f631edce102b02ccbdbfb18be1376a86b41373d.1586883286.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>