- Pat noticed that makedefs -z makes both vis_tab.[ch], but they could be
built by two makes via make -j, causing corruption
- make -j at top level failed for similar reasons, added several
dependencies to ensure a valid ordering
- these changes will cause extra things to be built if you "make"
individual targets at the top level, but have little effect it you're a
real power user and "make" the actual target in the right directory
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