Make sure we depend on the right name of mkdoc all the time, and that
the rules that need it (e.g. .def files) depend on it.
Reported-by: Jon Turney <jon.turney@dronecode.org.uk>
This allows building the libc & libm pages in parallel, and drops
the duplication in the subdirs with the chew/chapter settings.
The unused rules in Makefile.shared are left in place to minimize
noise in the change.
Mike Frysinger