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.
This simplifies the `cmMakefile::AddDefinition` method with bool value
overload to call the string based `cmMakefile::AddDefinition` method
with either an "ON" or "OFF" string.
Also the method is renamed to `cmMakefile::AddDefinitionBool`
