m4: use discovered Lua interpreter for its own version analysis.

* m4/ax_lua.m4 (AX_PROG_LUA): Use discovered Lua interpreter to
repor its own version number.
(_AX_LUA_CHK_VER): Use Lua code for version comparison rather
than depending on AX_COMPARE_VERSION macro, some awk and a slew
of sed.
* m4/ax_compare_version.m4: Remove.  Surplus to newly reduced
requirements.
* AUTHORS, README.md: Remove references to ax_compare_version.m4.
* NEWS: Update.

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