1. Go to "Tools" -> "Options" -> "Build & Run" -> "Kits".
2. Select a Kit.
3. In the "Environment" row, click "Change..."
   A dialog with an empty text edit will open. There is no explanation what you can do there. I found out myself that you'll have to type VARNAME=value to change an environment variable.
4. Try to use this for appending something to an existing environment variable.
   I did not find a way. I tried:
   PATH=%PATH%;C:\new\path (Windows-style)
   PATH=$PATH;C:\new\path (Unix-style)
   PATH+=C:\new\path (Qt-style)
   None of these worked as expected.

The "Edit Environment Changes" dialog should provide some information what the user should enter there. If there is one, it should also show the way to use existing environment variables. If you can't use existing environment variables, that would be another bug...