configure: enable Lua if found and continue without if not

Search for Lua if no --enable-lua/--disable-lua specified but continue
without if not found.

If --enable-lua is specified and Lua is not found then return error.

If --disable-lua is specified, then don't search for Lua.

Signed-off-by: Natanael Copa <ncopa@alpinelinux.org>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>