Make the docs system somewhat more user-friendly:

Make the docs system somewhat more user-friendly:
 - Configure now prints a summary of what output formats the docs can be built
 in using the utilities that it found and prints the names of the missing
 utilities for the formats that can't be built.
 - Add 'make all' that builds all the docs that can be built using the
 utilities that were found (the current 'make all' has been renamed to
 'make help' and is still the first target in the Makefile)
 - Add a few utility functions for autoconf