BUG: Fix get_property result for bad property

When a property does not exist we are supposed to return an empty value.
Previously if a property did not exist we just left the value of the
output variable unchanged.  This teaches CMake to remove the definition
of the output variable in this case.

Source/cmGetPropertyCommand.cxx

@@ -205,7 +205,14 @@ bool cmGetPropertyCommand::StoreResult(const char* value)
     }
   else // if(this->InfoType == OutValue)
     {
-    this->Makefile->AddDefinition(this->Variable.c_str(), value);
+    if(value)
+      {
+      this->Makefile->AddDefinition(this->Variable.c_str(), value);
+      }
+    else
+      {
+      this->Makefile->RemoveDefinition(this->Variable.c_str());
+      }
     }
   return true;
 }