Don't define rules based on PROGS if PROGS is empty.

Reviewed by:	sjg, ngie
MFC after:	1 week
Sponsored by:	EMC / Isilon Storage Division


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273186 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f