How to create variables for use in Xcode build settings values?

Question

XCode has a number of built-in variables that are used for convenience in Build Settings, for example "$(PRODUCT_NAME)" or "$(CURRENT_ARCH)".

Can I define my own variables? How / where?

Answer 1

In XCode 5 this has changed slightly.

1. Select the project or target in the left side of the editor
2. Go to the Editor menu in the top menu bar
3. Select "Add Build Setting" and then "Add User-Defined Build Setting"