cmProp: refactoring: transform alias in class

To handle safely the values used by CMake variables and properties,
introduce the class cmProp as a replacement from the simple pointer
to std::string instance.

Source/cmIncludeGuardCommand.cxx

@@ -4,6 +4,7 @@
 
 #include "cmExecutionStatus.h"
 #include "cmMakefile.h"
+#include "cmProperty.h"
 #include "cmStateDirectory.h"
 #include "cmStateSnapshot.h"
 #include "cmSystemTools.h"