specl: sync with upstream slingshot for SPECL_ENV improvements.

* slingshot: Sync with upstream.
* specs/specs.mk: Simplify accordingly.
* specs/spec_helper.lua (package.path, package.cpath): Set
according to slingshot recommendations.  `make distcheck` works
again now, even when Specl (>=13) is using a different lyaml
release from the system directories.

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