Having to duplicate configure tests in all supported Atheme release and
development branches, to enable contrib modules to be built reliably on
all of our supported platforms, was quickly becoming untenable.

Move the checks to contrib's own configure script, which necessitates
importing some build system things from Atheme proper.

This will not be backported to Atheme 7.1 or earlier; those versions are
no longer supported. If this submodule is bumped in those branches, the
contrib modules will no longer build, and make will likely fail.