docs/conf.py: Treat mathjax as fallback math renderer

Currently, math expressions using the "math::" directive or
the ":math:" role of Sphinx need the imgmath extension for proper
rendering in html and epub builds.
imgmath requires dvipng (and latex).
Otherwise, "make htmldocs" will complain of missing commands.

As a matter of fact, the mathjax extension is loaded by default since
Sphinx v1.8 and it is good enough for html docs without any dependency
on texlive packages.

Stop loading the imgmath extension for html docs unless requirements
for imgmath are met.

To find out whether required commands are available, add a helper
find_command(), which is a wrapper of shutil.which().

For epub docs, keep the same behavior of always loading imgmath.
Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Acked-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Link: https://lore.kernel.org/r/a6a877fc-dc93-2bda-a6d3-37001d99942a@gmail.com
[jc: Took out the writing of the math_renderer decision]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/conf.py

@@ -15,6 +15,18 @@
 import sys
 import os
 import sphinx
+import shutil
+
+# helper
+# ------
+
+def have_command(cmd):
+    """Search ``cmd`` in the ``PATH`` environment.
+
+    If found, return True.
+    If not found, return False.
+    """
+    return shutil.which(cmd) is not None
 
 # Get Sphinx version
 major, minor, patch = sphinx.version_info[:3]
@@ -106,7 +118,20 @@ else:
 autosectionlabel_prefix_document = True
 autosectionlabel_maxdepth = 2
 
-extensions.append("sphinx.ext.imgmath")
+# Load math renderer:
+# For html builder, load imgmath only when its dependencies are met.
+# mathjax is the default math renderer since Sphinx 1.8.
+have_latex =  have_command('latex')
+have_dvipng = have_command('dvipng')
+load_imgmath = ((have_latex and have_dvipng)
+                or (major == 1 and minor < 8)
+                or 'epub' in sys.argv)
+
+if load_imgmath:
+    extensions.append("sphinx.ext.imgmath")
+    math_renderer = 'imgmath'
+else:
+    math_renderer = 'mathjax'
 
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['_templates']