When MinGW is used slashes are used for dependencies
because ar.exe can't read rsp files with backslashes.
Many thx to Claus Klein for starting working on 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.
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.
Peter Kuemmel
Stephen Kelly
Nicolas Despres
Brad King
Peter Collingbourne