Implement cmGlobalGenerator::ComputeTargetObjects in the Ninja generator
to pre-compute all the object file names. Use the results during
generation instead of re-computing it later.
The confluence of the ninja-generator and target-include-directories
branches conspired to produce a nice little compiler error when
they were both merged into 'next'...
Yay for Continuous dashboards!
Generally these are only required in build statements, as Ninja wants
to be able to chop paths up. But it doesn't hurt to also try to use
them in command line arguments.