(1) Add new variable, XFREE86_VERSION, to specify which version of XFree86
(3 or 4) to depend to when USE_XLIB is set. XFREE86_VERSION defaults to 3
for now, but adventurous users can override it in /etc/make.conf. When
XFREE86_VERSION=3, USE_XLIB will add a dependency to x11/XFree86; when it
is set to 4, the dependency will be to x11/XFree86-4-libraries. When
XFREE86_VERSION=4, the PKG_IGNORE_DEPENDS and ALWAYS_BUILD_DEPENDS hacks
to avoid messing with XFree86 are turned off.