Fix CPP cleanup

src/csharp/Grpc.Tools/build/_protobuf/Google.Protobuf.Tools.targets

@@ -100,6 +100,7 @@
           Condition=" '@(Protobuf)' != '' "
           DependsOnTargets=" Protobuf_BeforeCompile;
                              Protobuf_ResolvePlatform;
+                             _Protobuf_SetProtoRoot;
                              _Protobuf_SelectFiles;
                              Protobuf_PrepareCompile;
                              _Protobuf_AugmentLanguageCompile;
@@ -115,27 +116,33 @@
           DependsOnTargets="Protobuf_Compile"
           Condition=" '$(Language)' == 'C#' " />
 
-  <Target Name="_Protobuf_SelectFiles">
+  <Target Name="_Protobuf_SetProtoRoot">
     <!-- Guess .proto root for the files. Whenever the root is set for a file explicitly,
          leave it as is. Otherwise, for files under the project directory, set the root
          to "." for the project's directory, as it is the current when compiling; for the
-         files outside of project directory, use each .proto file's directory as the root. -->
+         files outside of project directory, use each .proto file's directory as the root 
+		 or Protobuf_ProtoRoot if set. -->
     <FindUnderPath Path="$(MSBuildProjectDirectory)"
                    Files="@(Protobuf->WithMetadataValue('ProtoRoot',''))">
       <Output TaskParameter="InPath" ItemName="_Protobuf_NoRootInProject"/>
       <Output TaskParameter="OutOfPath" ItemName="_Protobuf_NoRootElsewhere"/>
     </FindUnderPath>
+	
+    <ItemGroup>
+      <Protobuf>
+		<!-- In-project files will have ProtoRoot='.'. -->
+        <ProtoRoot Condition=" '%(ProtoRoot)' == '' and '@(_Protobuf_NoRootInProject)' != '' ">.</ProtoRoot>
+		<!-- Out-of-project files will have respective ProtoRoot='%(RelativeDir)'. -->
+        <ProtoRoot Condition=" '%(ProtoRoot)' == '' and '@(_Protobuf_NoRootElsewhere)' != '' ">%(RelativeDir)</ProtoRoot>
+      </Protobuf>
+    </ItemGroup>
+  </Target>
+
+  <!-- Select files that should be compiled. -->
+  <Target Name="_Protobuf_SelectFiles">
     <ItemGroup>
       <!-- Files with explicit metadata. -->
-      <Protobuf_Compile Include="@(Protobuf->HasMetadata('ProtoRoot'))" />
-      <!-- In-project files will have ProtoRoot='.'. -->
-      <Protobuf_Compile Include="@(_Protobuf_NoRootInProject)">
-        <ProtoRoot>.</ProtoRoot>
-      </Protobuf_Compile>
-      <!-- Out-of-project files will have respective ProtoRoot='%(RelativeDir)'. -->
-      <Protobuf_Compile Include="@(_Protobuf_NoRootElsewhere)">
-        <ProtoRoot>%(RelativeDir)</ProtoRoot>
-      </Protobuf_Compile>
+      <Protobuf_Compile Include="@(Protobuf)" />
       <!-- Remove files not for compile. -->
       <Protobuf_Compile Remove="@(Protobuf_Compile)" Condition=" '%(ProtoCompile)' != 'true' " />
       <!-- Ensure invariant Source=%(Identity). -->
@@ -329,6 +336,7 @@
   <Target Name="Protobuf_Clean"
           Condition=" '@(Protobuf)' != '' "
           DependsOnTargets=" Protobuf_BeforeClean;
+                             _Protobuf_SetProtoRoot;
                              Protobuf_PrepareClean;
                              _Protobuf_CoreClean;
                              Protobuf_AfterClean" />