doc: Fix the error PDFs are needed but doxygen isn't available
dist: don't automatically rebuild doxygen on each make dist

Warn if the doc doesn't exist, or just use what's already there.

contrib/dist/make_dist_tarball takes care of removing/recreating the doc,
and the doc is already correctly rebuilt in most cases, it is built by
make dist when not already available.
No need to keep complex rules for always rebuilding the doc just in case
somebody ever builds a tarball manually and ends up containing an outdated
doc for some reason.

Remove --enable-doxygen from make distcheck now that things are simple.