Use cascading-if for per-config test and install code

When generating per-config blocks in test and install scripts replace
the form

  IF()
    # config == A
  ENDIF()
  IF()
    # config == B
  ENDIF()

with

  IF()
    # config == A
  ELSEIF()
    # config == B
  ELSE()
    # no config matches
  ENDIF()

for clarity and to support the else() case cleanly.

Source/cmScriptGenerator.h

@@ -65,6 +65,8 @@ protected:
   virtual void GenerateScriptForConfig(std::ostream& os,
                                        const char* config,
                                        Indent const& indent);
+  virtual void GenerateScriptNoConfig(std::ostream&, Indent const&) {}
+  virtual bool NeedsScriptNoConfig() const { return false; }
 
   // Test if this generator does something for a given configuration.
   bool GeneratesForConfig(const char*);