specl: start adding specl specifications for zlisp.

Start with the previously untested (previously untestable!)
`where-is` command.
* configure.ac (AC_PATH_PROG): Probe for a specl command.
* bootstrap.conf (buildreq): Add specl.  Note, we need bugfixes
from an unreleased Specl at the moment.
(slingshot_files): Add bulid-aux/specl.mk.
* lib/zmacs/specs/spec_helper.lua: New file. Matchers and helpers
for running zmacs and checking buffer and minibuf results.
* lib/zmacs/specs/zlisp_bind_spec.yaml (where-is): Specify
behaviour of `where-is` command.
* lib/zmacs/specs/specs.mk: New file. Declare specl spec files.
* local.mk: Include it.
* travis.yml.in (script): Check specs.
* .travis.yml: Regenerate.

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