Build website incrementally (and faster if you have the CPU cores)

This is done by replacing www/generate.sh with a Makefile. Benefits:

- Makes builds incremental, meaning that only the minimum number of
  markdown files will be converted to HTML during a build. The previous
  scheme always generated a new HTML file for every markdown file,
  which is a big waste of time if only 1 or 2 markdown files have been
  changed.

- Allows for much faster builds through concurrent jobs (e.g., via
  "make -j4").  On my 4-core machine, my average build time for the
  website with generate.sh was just over 26 seconds; with "make -j4",
  it was 13 seconds.

- Avoids portability issues with find(1) in generate.sh, which I was
  encountering on OpenBSD.

A note on portability: unlike GNU Make, OpenBSD's Make does not have
the "$(shell [commands])" construct, so we don't use that. Instead we
use "!= [commands]", which is supported by both.

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