turn off maintainer mode

Currently, we use the default Autotools "maintainer mode" setting, which I believe is enabled. The practical consequence, AFAICT, is that configure gets re-run at weird times, and we can't put make clean in autogen.sh.

This behavior makes me nervous; I feel we should disable maintainer mode by default with AM_MAINTAINER_MODE([disable]) in configure.ac. However, I couldn't find an obvious consensus immediately on the interwebs.

So, this issue is to figure out what to do and do it.

Follow-up to PR #589.

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