make pdfdocs fails with v4.9-rc3

[Jim Davis]

Running make pdfdocs on a Ubuntu 16.04 system fails with

Makefile:58: recipe for target 'media.pdf' failed
make[2]: *** [media.pdf] Error 1
Documentation/Makefile.sphinx:79: recipe for target 'pdfdocs' failed
make[1]: *** [pdfdocs] Error 2
Makefile:1442: recipe for target 'pdfdocs' failed
make: *** [pdfdocs] Error 2

It's not clear from the voluminous (>5MB) output from make pdfdocs
exactly which stage is failing, and there are some pdf files produced
in output/latex.

This is with sphinx, rst2pdf, and texlive installed from the Ubuntu
repositories.

-- 
Jim