make default install component name configurable

Until now an unnamed component was always named "Unspecified".
Now this name is taken from the new cmake variable CMAKE_INSTALL_DEFAULT_COMPONENT_NAME,
which is initialized to "Unspecified". But it can now be set to something
project-specific, per directory

Alex

Source/cmInstallCommand.h

@@ -341,6 +341,8 @@ private:
                          const std::vector<std::string>& relFiles,
                          std::vector<std::string>& absFiles);
   bool CheckCMP0006(bool& failure);
+
+  std::string DefaultComponentName;
 };