configury: fix 'make distcheck'.

* GNUmakefile (MKROCKSPECS): Spell mkrockspecs without .lua suffix.
* specl/specl.mk (EXTRA_DIST): Likewise.
* Makefile.am (EXTRA_DIST): Spell specl-rockspec.lua as
rockspecs.lua.

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