Some large applications can be built in a number of configurations, adding functionality if one of a number of libraries or applications is available. Since not all users want those libraries or applications, the ports system provides hooks that the port author can use to decide which configuration should be built. Supporting these properly will make uses happy, and effectively provide 2 or more ports for the price of one.
The easiest of these to use is WITHOUT_X11. If the port can be built both with and without X support, then it should normally be built with X support. If WITHOUT_X11 is defined, then the version that does not have X support should be built.
Various parts of GNOME have such knobs, though they are slightly more difficult to use. The variables to use in the Makefile are WANT_* and HAVE_*. If the application can be built both with or without one of the dependencies listed below, then the Makefile should set WANT_PKG, and should build the version that uses PKG if HAVE_PKG is defined.
The WANT_* variables currently supported this way are WANT_GLIB, WANT_GTK, WANT_ESOUND, WANT_IMLIB, and WANT_GNOME.