Cleanup cppunit detection and use

The cppunit-config tool has apparently been replaced by pkg-config .pc
file years ago and is now in the process of being removed from some OS.
Notably Fedora.

Which means our present way of detecting it for use by "make check" will
increasingly fail.

This converts configure.ac to using the pkg-config method of detection
and updates the --with-cppunit-basedir parameter to --without-cppunit
matching our naming and usage scheme for other similar options. If a
=PATH is explicitly provided cppunit is assumed to exist at that
location without configure-time checking.