--- virtualx.eclass.orig	2011-08-05 13:14:44.000000000 +1200
+++ virtualx.eclass	2011-08-05 13:28:04.000000000 +1200
@@ -93,9 +93,19 @@
 
 	# If $DISPLAY is not set, or xhost cannot connect to an X
 	# display, then do the Xvfb hack.
-	if [[ -n ${XVFB} && -n ${XHOST} ]] && \
+	if [[ -n ${XHOST} ]] && \
 			( [[ -z ${DISPLAY} ]] || ! (${XHOST} &>/dev/null) ) ; then
 		debug-print "${FUNCNAME}: running Xvfb hack"
+
+		# On prefix, we require Xvfb to be provided by the host
+		if use prefix ; then
+			if test -z "${XVFB}" ; then
+				eerror "On prefix, we require Xvfb to be provided by the host."
+				eerror "Please install the appropriate pacakge for your system."
+				die "Unable to find 'Xvfb' for X11-based build/tests."
+			fi
+		fi
+
 		export XAUTHORITY=
 		# The following is derived from Mandrake's hack to allow
 		# compiling without the X display