BUG: Fix/cleanup custom commands and custom targets. Make empty comment strings work. Fix ZERO_CHECK target always out of date for debugging. Fix Makefile driving of custom commands in a custom target. Fix dependencies on custom targets not in ALL in VS generators.

Source/cmCustomCommand.h

@@ -70,6 +70,7 @@ private:
   std::vector<std::string> Outputs;
   std::vector<std::string> Depends;
   cmCustomCommandLines CommandLines;
+  bool HaveComment;
   std::string Comment;
   std::string WorkingDirectory;
   bool EscapeAllowMakeVars;