SWIG: slightly more portable automatic dependency tracking

The SWIG bindings use automatic dependency tracking for SWIG-generated files, but Automake doesn't make it easy to link this into its usual automatic dependency tracking framework, so the code to do this relied on some Automake internals. It looks like these have changed in Automake > 1.16 (checked out from git, commit 596e9e130f38ae068ac6150b2d62eafd544900f6). I've now reworked the SWIG automatic dependency tracking code to be hopefully slightly less dependent on Automake internals - at worst it should now issue a more helpful error message if/when it breaks.