ENH: Added post-build rules to executables and libraries.  Generalized AppendLibDepend method to AppendAnyDepend.  This takes most of the functionality of AppendCustomDepend too, and generalized jump-and-build to executables.

ENH: Implemented utility targets.  This involved pulling part of the custom command rule implementation out into shared methods.

BUG: Need to include header <new> to use placement new syntax.  Really this should be fixed by replacing the stream buffer to set a new string instead of reconstructing the object, but this will require quite a bit of work to do portably.

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.