docs/conf.py: Respect env variable SPHINX_IMGMATH

docs/conf.py: Respect env variable SPHINX_IMGMATH

On some distros with coarse-grained packaging policy, dvipng is
installed along with latex.  In such cases, math rendering will
use imgmath by default.  It is possible to override the choice by
specifying the option string of "-D html_math_renderer='mathjax'"
to sphinx-build (Sphinx >= 1.8).

To provide developers an easier-to-use knob, add code for an env
variable "SPHINX_IMGMATH" which overrides the automatic choice
of math renderer for html docs.

    SPHINX_IMGMATH=yes : Load imgmath even if dvipng is not found
    SPHINX_IMGMATH=no  : Don't load imgmath (fall back to mathjax)

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Acked-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Link: https://lore.kernel.org/r/5a582b2b-d51c-a062-36b2-19479cf68fab@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>