pgxs regression test support

Is pgxs intended to support regression tests similar to contrib? The
docs list REGRESS as a valid variable. If so, how do you use it?

# make -f Makefile.pgxs installcheck
Makefile.pgxs:49: warning: overriding commands for target `installcheck'
/usr/local/pgsql-dev/lib/pgxs/src/makefiles/pgxs.mk:228: warning:
ignoring old commands for target `installcheck'
make -C
/usr/local/pgsql-dev/lib/pgxs/src/makefiles//../../src/test/regress
pg_regress
make: ***
/usr/local/pgsql-dev/lib/pgxs/src/makefiles//../../src/test/regress: No
such file or directory. Stop.
make: *** [submake] Error 2

I see the same error with, e.g. contrib/tablefunc.

The "warning: overriding commands for target `installcheck'" is expected
(BTW, if anyone knows how I can support two different expected outputs
depending on Postgres version, without overriding installcheck, I'd love
to hear it).

Thanks,

Joe