Fix handling of runtimeconfig files in incremental builds

Fixes dotnet#471

Author: dsplaisted

TestAssets/TestProjects/AppWithLibrary/TestApp/runtimeconfig.template.json

@@ -0,0 +1,5 @@
+{
+  "configProperties": {
+    "System.GC.Server": true
+  }
+}

src/Tasks/Microsoft.NET.Build.Tasks/build/Microsoft.NET.Sdk.targets

@@ -124,8 +124,14 @@ Copyright (c) .NET Foundation. All rights reserved.
                                        PlatformLibraryName="$(MicrosoftNETPlatformLibrary)"
                                        UserRuntimeConfig="$(UserRuntimeConfig)">
 
-      <Output TaskParameter="FilesWritten" ItemName="FileWrites" />
     </GenerateRuntimeConfigurationFiles>
+    
+    <ItemGroup>
+      <!-- Do this in an ItemGroup instead of as an output parameter of the GenerateDepsFile task so that it still gets added to the item set
+          during incremental builds when the task is skipped -->
+      <FileWrites Include="$(ProjectRuntimeConfigFilePath)" Condition="Exists('$(ProjectRuntimeConfigFilePath)')"/>
+      <FileWrites Include="$(ProjectRuntimeConfigDevFilePath)" Condition="Exists('$(ProjectRuntimeConfigDevFilePath)')"/>
+    </ItemGroup>
 
   </Target>