Negative reasoning like `allBut` is a bad idea with an open world of
platforms. Concretely, if we add a new, quite different sort of
platform, existing packages with `allBut` will claim they work on it
even though they probably won't.
See #32386 -- while Mono in general should build correctly with parallel
building, it seems the 4.4 branch has broken.
Instead, allow parallel build support to be overridden by individual
versions, and default to true.
Signed-off-by: Austin Seipp <aseipp@pobox.com>
