configure: luac: autodetect host if not provided

When building the luac contrib, it is built at $(PREFIX)/../bin/$(cc -dumpmachine)-luac

When configuring, if --host is not provided, the script looks for luac at $(PREFIX)/../bin/-luac as $host_alias is empty, failing to lookup luac.

Use (cc -dumpmachine) to look for the LUA byte compiler when host is not provided.