This commit allows to override a global or target-wide 'job pool'
property on a per-source basis.
It modifies the Ninja generator to first look into the source's
properties, and only use the target properties as a fallback.
The new `source` parameter to
`cmNinjaTargetGenerator::addPoolNinjaVariable` may be null, e.g. when
the function is called in context where there is no source (e.g. a
linking task).
Closes: #23994
3a9d56d2c7