How can I get T4 files to build in Visual Studio?

When I build my C# solution, the .tt files will not create the .cs file outputs.

But if I right-click the .tt files one at a time in Solution Explorer, and select "Run Custom Tool", the .cs file is generated, so the build tool setting is correct.

What do I do to get the overall solution build to force the custom tool to run on the .tt files?