Ninja Multi-Config: Enable relink diagnostic message

We already have a message for the "Ninja" generator, but it applies
to the "Ninja Multi-Config" generator too.

Source/cmGeneratorTarget.cxx

@@ -2003,8 +2003,7 @@ bool cmGeneratorTarget::NeedRelinkBeforeInstall(
   // this target must be relinked.
   bool have_rpath =
     this->HaveBuildTreeRPATH(config) || this->HaveInstallTreeRPATH(config);
-  bool is_ninja =
-    this->LocalGenerator->GetGlobalGenerator()->GetName() == "Ninja";
+  bool is_ninja = this->LocalGenerator->GetGlobalGenerator()->IsNinja();
 
   if (have_rpath && is_ninja) {
     std::ostringstream w;

Source/cmGlobalGenerator.h

@@ -438,6 +438,8 @@ public:
 
   virtual bool IsVisualStudio() const { return false; }
 
+  virtual bool IsNinja() const { return false; }
+
   /** Return true if we know the exact location of object files.
       If false, store the reason in the given string.
       This is meaningful only after EnableLanguage has been called.  */

Source/cmGlobalNinjaGenerator.h

@@ -170,6 +170,8 @@ public:
 
   static std::string GetActualName() { return "Ninja"; }
 
+  bool IsNinja() const override { return true; }
+
   /** Get encoding used by generator for ninja files */
   codecvt::Encoding GetMakefileEncoding() const override;