turn --with-json-audit into global --with-json

turn --with-json-audit into global --with-json

Fold the build option --with-json-audit into the toplevel wscript
to reflect the fact that JSON support is no longer local to the
audit subsystem.

Signed-off-by: Philipp Gesang <philipp.gesang@intra2net.com>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>