ENH: Improve new error/warning message generation

- Add cmListFileBacktrace to record stack traces
  - Move main IssueMessage method to the cmake class instance
    (make the backtrace an explicit argument)
  - Change cmMakefile::IssueMessage to construct a backtrace
    and call the cmake instance version
  - Record a backtrace at the point a target is created
    (useful later for messages issued by generators)

Source/cmListFileCache.h

@@ -57,11 +57,15 @@ struct cmListFileContext
   long Line;
 };
 
+std::ostream& operator<<(std::ostream&, cmListFileContext const&);
+
 struct cmListFileFunction: public cmListFileContext
 {
   std::vector<cmListFileArgument> Arguments;
 };
 
+class cmListFileBacktrace: public std::vector<cmListFileContext> {};
+
 struct cmListFile
 {
   cmListFile()