The pre-defined 'console' pool is a non-buffered pool that runs with a
depth of 1. CMake re-run cannot be run concurrently and it will
eventually output something. A non-buffered pool allows to get it as
soon as possible
Also, generate the minimal required version of Ninja in the build file.
Until now the cmCustomCommandGenerator was used only to compute the
command lines of a custom command. Generalize it to get the comment,
working directory, dependencies, and outputs of custom commands. Update
use in all generators to support this.
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.