meson: add 'system-lua' project option to toggle the bundled lua

By default system-lua=false, so the bundled version is built.

The default_library for the lua subproject is also set to static
now, so that we don't need to install liblua and mess up the system.
For existing build trees, this needs to be switched manually now with
-Dlua:default_library=static (or just wipe the build dir and start clean)