Add a virtual cmGlobalGenerator::ComputeTargetObjects method invoked
during cmGeneratorTarget construction. Implement it in the Makefile
generator to pre-compute all object file names for each target. Use
the results during generation instead of re-computing it later.
Some per-target information and logic is common to all generators.
Some of that information is currently stored in cmTarget but that
should be reserved for the configure step. Create a class to hold
per-target information for generators. On construction classify
sources from the target and store them in separate members. This
classification is already implemented separately in each generator.
Brad King