Fix make dist during make distcheck

It wants to generate doxygen docs, but it doesn't by default
since it builds from a tarball. Force --enable-doxygen to fix that.

By the way, no need to double-check that doc/readme were generated
before building the tarball. config/distscript.csh is called on dist
and it does things as expected.

Also add a better error message when make dist fails because
PDFs do not exist yet, and doxygen isn't available.

Ideally, we'd just remove distscript.csh to make things much simpler
but I don't want to break more things today.