- Remove FreeBSD 4.x support (USE_GCC=3.4+ and files/extra-patch-*).
- Add USE_XLIB when using USE_GL, since it doesn't imply it anymore.
- Use ~/.ioquake3 for writing instead of original ~/.q3a, because the new
configuration files have more variables, and would be removed if overwriten
by another engine. Note it in pkg-message.
- Install README.