Change native-compile to no-op if no native projects found

- #93

build/_k-standard-goals.shade

@@ -46,7 +46,9 @@ default Configuration='Release'
   var nativeProjects ='${Files.Include(Path.Combine(BASE_DIR, "src", "**", "*.vcxproj"))}'
 
   @{
-      if(!File.Exists(msbuild))
+      if (nativeProjects.Any())
+      {
+        if (!File.Exists(msbuild))
         {
             Log.Warn("msbuild version 12 not found. Please ensure you have the VS 2013 C++ SDK installed.");
             Environment.Exit(1);
@@ -60,6 +62,7 @@ default Configuration='Release'
             }
         }
       }
+  }
 
   copy sourceDir='${Path.GetDirectoryName(project)}' include='bin/**/' outputDir='${Path.Combine(BUILD_DIR, Path.GetFileNameWithoutExtension(project))}' overwrite='${true}' each='var project in nativeProjects'