It's especially annoying to see this:

  --enable-perf           Build performance measurement tools [default=yes].
  --disable-eventfd       disable eventfd [default=no]
  --enable-curve-keygen   Build curve key-generation tool [default=yes].

Solution: all options should explain the non-default case. Also
the language should be enable/disable, with/without, rather than
yes/no. E.g. '--without-docs'.