With Ninja 1.10 we run the cleandead and recompact tools after
generation. These require that `build.ninja` be loadable. Update the
`CustomCommandJobPool` case to define the referenced job pools to make
`build.ninja` loadable.
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