configury: don't fail on missing make fragments.

* Makefile.am: Use `-include` so that this file is useful in a
parent project that doesn't have all the listed fragments.
* local.mk: Put project specific fragment includes here.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
configury: support minimum specl version assertion.

* configure.ac (SPECL_MIN): Set and substitute a sensible minimum.
* build-aux/specl.mk (specl-check-local): Bail out if specl is too
old.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
travis: make additional bootstrap rocks configurable.

* configure.ac (EXTRA_ROCKS): Set to lyaml, which is always
needed to run slingshot mkrockspecs.
* travis.yaml.in: Use @EXTRA_ROCKS@ instead of hard-coding.

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