Make the documentation build reproducibly

Make the documentation build reproducibly

Whilst working on the Reproducible Builds effort [0] I noticed that
json-c could not be built reproducibly.

This is because it used the full, absolute path name as an (sanitised)
input to a filename, resulting in some binary package containing, for
example:

  /usr/share/doc/libjson-c-dev/html/md__build_1st_json-c-0_815_issues_closed_for_0_813.html
                                        ^^^^^^^^^^^^^^^^^^^^^^
or

  /usr/share/doc/libjson-c-dev/html/md__build_2_json-c-0_815_2nd_issues_closed_for_0_813.html
                                        ^^^^^^^^^^^^^^^^^^^^^^^^

These differing values are based on the path in which json-c is built. This was
originally filed in Debian as #966657 [1].

 [0] https://reproducible-builds.org/
 [1] https://bugs.debian.org/966657