Fix distcheck by dropping use of the now-obsolete gtkdoc-mktmpl.

In preparation for the upcoming 1.16 release, I've made a few changes to
get distcheck to pass.  I also updated it to run on Ubuntu 18.04, but
found that on newer distros distcheck won't run due to missing
gtkdoc-mktmpl, which has been deprecated upstream for some time.  The
patch below disables everything that references it, and enables
distcheck to finish successfully.

Unfortunately, this probably regresses portions of our document
generation, and thus will need some reimplementation work.  Anyone got
time to investigate a better solution for this?