configury: bump release version to 6.

* configure.ac (AC_INIT): Bump release version to 6.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
mkrockspecs: allow "builtin" builds even without --module-dir.

* src/mkrockspecs.in: Don't switch to autoconf builds for the
main rockspec if the default.build.modules is set in
rockspec.conf.
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>