The problem is that Sphinx can only read input files from a single
directory. In VPATH builds, the source and build trees are separate,
and the changes.rst went into the build tree.
This patch solves the issue by using cfunc as the Sphinx default role.
CHANGES is preprocessed to convert json_*() function names to Sphinx
:cfunc: cross references. This is to keep CHANGES more readable in
both plain text and HTML.
