Provide a way for custom commands and targets to set the pool variable
of the ninja build statement. Setting `JOB_POOL` is not compatible with
`USES_TERMINAL`, which implies the `console` pool.
The option is silently ignored with other generators.
Closes: #18483
This enables configuration of build parallelism with the Ninja generator by
passing arguments on the command line to CMake. For example,
cmake \
'-DCMAKE_JOB_POOLS:STRING=compile=5;link=2' \
-DCMAKE_JOB_POOL_COMPILE:STRING=compile \
-DCMAKE_JOB_POOL_LINK:STRING=link \
~/src/MyProject
