ENH: Unified WriteDependRules, WriteBuildRules, WriteCleanRules, and the future WriteInstallRules into a single WritePassRules method.  Also added WriteTargetDependsRule and WriteTargetCleanRule methods to unify writing of depend and clean rules for each target.

ENH: Moved code that checks output path variables to separate ConfigureOutputPaths method.  Needed to provide access to the same code from a subclass.

ENH: Generalized driver targets and subdirectory traversal rules.  The implementations of all, depend, build, clean, install, etc. now follow a common framework.

ENH: Re-implemented cmGeneratedFileStream to look like a real stream and replace the destination file atomically.  This will avoid problems with the process being terminated while generating a file.