Emscripten and OpenBSD exit out of the build script early. Since
02014b06c1a3 ("c-b: Turn `mem-unaligned` from a feature to a cfg"), this
meant that the exit happened before all `rustc-check-cfg`s had been
emitted.
Rework the logic so these only skip the C build rather than the rest of
configuration.
Trevor Gross
0a5785734c