lib: build-xserver.sh: support building extra dependencies of xserver

set $XORG_EXTRA_DEPS (usually in target config) to list of extra packages
built right before the Xserver.

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>

lib/build-xserver.sh

@@ -32,6 +32,11 @@ build_package xorg-util-macros
 build_package xorgproto
 build_package xorg-font-util
 build_package libdrm
+
+for i in $XORG_EXTRA_DEPS ; do
+    build_package $i
+done
+
 build_package xserver
 build_package xkeyboard-config
 build_package xkbcomp