configury: use static specs/spec_helper.lua.

No need to generate this file just to substitute @LUA@, just
use os.getenv "LUA" in a static file instead.
* specs/spec_helper.lua.in: Move from here...
* specs/spec_helper.lua: New file. ...to here.
(LUA): Set from LUA environment, or call `which lua` or just rely
on path search for "lua".
* .gitignore: Remove specs/spec_helper.lua.
* configure.ac (AC_CONFIG_FILES): Remove specs/spec_helper.lua
generator.
* specs/specs.mk (specs_path, spec-check-local): Remove.
(SPECL_ENV): Simplify.
(EXTRA_DIST): Adjust.
* specs/io_spec.yaml (process_files, readlines, slurp)
(writelines): Don't rely on specs/spec_helper.lua being in the
builddir, in case of VPATH builds.

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