mkrockspecs: accept and export luajit as LUA.

* src/merge-sections.in, src/mkrockspecs.in
(shell re-execute preamble): If LUA is not already in the
environment, accept a binary called luajit from the path, and
in any case re-export the discovered binary.
* build-aux/merge-sections: Regenerate.

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