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

doc/Doxyfile.in

@@ -152,7 +152,7 @@ FULL_PATH_NAMES        = YES
 # will be relative from the directory where doxygen is started.
 # This tag requires that the tag FULL_PATH_NAMES is set to YES.
 
-STRIP_FROM_PATH        =
+STRIP_FROM_PATH        = @CMAKE_SOURCE_DIR@
 
 # The STRIP_FROM_INC_PATH tag can be used to strip a user-defined part of the
 # path mentioned in the documentation of a class, which tells the reader which