[CMake] Ninja + parallel linking

[Robert Dailey]

I notice on ninja builds generated via CMake 3.6, the link step blocks
compilation threads. What I expect is that if I have 32 threads
available for compiling, those are also shared via the link step. In
other words, if I reach a link step, I want it to consume 1 thread and
the other 31 threads start compiling translation units for the next
library.

Is there a reason it isn't behaving as I describe? Is there a setting
or CMake option I can set to enable this?