--- Source/WebCore/GNUmakefile.list.am.old	2013-02-24 19:01:51.000000000 +0100
+++ Source/WebCore/GNUmakefile.list.am	2013-02-24 19:03:41.000000000 +0100
@@ -5949,6 +5949,9 @@
 webcore_sources += \
 	Source/WebCore/plugins/PluginPackageNone.cpp \
 	Source/WebCore/plugins/PluginViewNone.cpp
+# in quartz mode include cairo widgetbackingstore implementation also
+webcoregtk_sources += \
+	Source/WebCore/platform/cairo/WidgetBackingStoreCairo.cpp
 endif # END TARGET_WIN32
 endif # END TARGET_X11