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This avoids building the doc on each build, which is slow and most
likely not wanted.  So, move the doc from the "all" target to the "doc"
one, and hook on dist too.

This also fixes distributing the documentation by including it in the
dist tarballs.

This doesn't work for real yet because ValaDoc depends on valac >= 0.11
and < 0.13 but we depend on valac >= 0.13.