Use non-standard but convenient AX_SUBDIRS_CONFIGURE() macro instead of
the standard AC_CONFIG_SUBDIRS() for libtiff, in order to allow passing
extra options to this sub-configure script without affecting any other
sub-configure ones and also without wrongly caching these options as
part of the main configure options, as happened before.
This notably fixes the problem with running "config.status", which
complained about unknown "--disable-jbig" and similar options, as they
were cached by the previous configure run.
We could also manually remove these options from ac_configure_args, but
doing it like this seems cleaner and simpler.
