Waive the USE Flags

The core concept around which Portage operates is the idea of USE flags. Each software package defines its dependencies through these USE flags. For instance, if an application can be configured to use widget library "widg" but can also, optionally, use widget library "uberwidg", it would define these dependencies through these two flags. Perhaps widg would be a required dependency, whereas uberwidg may be an optional dependency. There are general, system-type USE flags that are used by all packages, and there are USE flags specific to individual packages.

You can think of them as environment variable equivalents to an application's configuration variables. If your application can use the GD graphics library, and its configure script has a "--with-gd" option, it probably has a gd USE option.