gdk/gdkwindow.c, gtk/gtkwidget.c: Include fallback-c89.c

Both of them started to make use of round(), a C99 function.  So, include
fallback-c89.c to provide a fallback implementation for round() for
compilers that don't have round()

https://bugzilla.gnome.org/show_bug.cgi?id=694339

gdk/gdkwindow.c

@@ -42,6 +42,9 @@
 
 #include <math.h>
 
+/* for the use of round() */
+#include "fallback-c89.c"
+
 #undef DEBUG_WINDOW_PRINTING
 
 

gtk/gtkwidget.c

@@ -66,6 +66,9 @@
 #include "gtktypebuiltins.h"
 #include "a11y/gtkwidgetaccessible.h"
 
+/* for the use of round() */
+#include "fallback-c89.c"
+
 /**
  * SECTION:gtkwidget
  * @Short_description: Base class for all widgets