install: Thread message level setting through internal API

Create a cmInstallGenerator::MessageLevel enumeration for future use in
specifying install message verbosity.  Thread values of the type through
constructors and save the value as a member of cmInstallGenerator.
Use only a "MessageDefault" value for now.

Source/cmInstallProgramsCommand.cxx

@@ -93,11 +93,13 @@ void cmInstallProgramsCommand::FinalPass()
   std::string no_component = this->Makefile->GetSafeDefinition(
                                        "CMAKE_INSTALL_DEFAULT_COMPONENT_NAME");
   std::vector<std::string> no_configurations;
+  cmInstallGenerator::MessageLevel message =
+    cmInstallGenerator::MessageDefault;
   this->Makefile->AddInstallGenerator(
     new cmInstallFilesGenerator(this->Makefile, this->Files,
                                 destination.c_str(), true,
                                 no_permissions, no_configurations,
-                                no_component.c_str(), no_rename));
+                                no_component.c_str(), message, no_rename));
 }
 
 /**