nmake/jom: Only warn about bad VS environment if compiler not found.

nmake and jom generators no longer warn about missing INCLUDE/LIB
environment variables unless the C/CXX compiler cannot be found.  This
is useful if the user does not want to use these generators with the
Visual C++ compiler, or they do not want to enable any language.

Source/cmGlobalGenerator.h

@@ -374,6 +374,8 @@ protected:
   void SetLanguageEnabledFlag(const std::string& l, cmMakefile* mf);
   void SetLanguageEnabledMaps(const std::string& l, cmMakefile* mf);
   void FillExtensionToLanguageMap(const std::string& l, cmMakefile* mf);
+  virtual void PrintCompilerAdvice(std::ostream& os, std::string const& lang,
+                                   const char* envVar) const;
 
   virtual bool ComputeTargetDepends();
 
@@ -462,8 +464,6 @@ private:
 
   virtual void ForceLinkerLanguages();
 
-  virtual void PrintCompilerAdvice(std::ostream& os, std::string const& lang,
-                                   const char* envVar) const;
   void CheckCompilerIdCompatibility(cmMakefile* mf,
                                     std::string const& lang) const;