Add distprep target to take some of the job of the release_prep script.

The latter updated accordingly. Also add `dist' and `distcheck' targets
to play with, but caveat packager.

Updated backend/bootstrap and backend/parser makefile to make them
marginally builddir aware and fix the usual set of things.

Add rule to automatically remake config.h dependent on config.h.in and
config.status. (Adopted from Autoconf manual and about every other
package.) On a good day we should now have a complete and accurate set
of dependencies throughout everything.