Having looked at this again, the current patch is just far bigger than it needs to be and tries to do too much, not to mention being rather out of date now.

So, here's a much less ambitious, much simpler patch with many fewer ways it can go wrong (but also not quite as close a match to using Makefile).  Instead of only allowing specific targets, the script now only specifies non-Sphinx-builder targets (like clean, check, help, htmlview, and serve) and assumes that any first argument it doesn't recognize is supposed to be a Sphinx builder.  "htmlhelp" is still special-cased in the "build" target.  Not listing all the Sphinx builder targets explicitly means that builders like "dirhtml", "pickle", or "json", or any builders added in future Sphinx versions will just work without our having to do anything for them.