How to set environment variables in Visual Studio 2012?

Question

How do I set environment variables in Visual Studio 2012?

For example, I need to set this:

$(MyDir) = "c:\my_dir"

Answer 1

Open the project in a text editor, and into the top existing tag add

<PropertyGroup>
  <MyDir>c:\my_dir</MyDir>
  ...(leave existing ones here)...
</PropertyGroup>

There's probably some way to do this in the UI, but I only ever edit the files by hand nowadays. And its a different UI for every language(!)