release: make sure rockspecs are generated before emails.

* build-aux/release.mk (mail): Add dependency on rockspecs.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
mkrockspecs: define spec.build before dereferencing it.

* src/mkrockspecs.in: If spec.build is not set before accessing
default to the empty table.

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