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.
Just enough to reach the BuildMacContentDirectory method and the
NeedRelinkBeforeInstall methods.
In the future, those methods can be moved to cmGeneratorTarget.
To properly track the usage of dependencies that are generated at
compile time as the side effect of other build steps we need
to make the WriteBuild and WritePhonyBuild commands non static
Brad King
Stephen Kelly
Robert Maynard
Peter Collingbourne