Mention X11 build requirements (since XInput2 is something of
an actual requirement, as opposed to 'newer than R5').
GdkPixbuf @GDK_PIXBUF_REQUIRED_VERSION@, ATK @ATK_REQUIRED_VERSION@
and cairo @CAIRO_REQUIRED_VERSION@.
+ - Each GDK backend has its own backend-specific requirements. For
+ the X11 backend, X11 R6 and XInput version 2 (as well as a number
+ of other extensions) are required.
+
- gobject-introspection @INTROSPECTION_REQUIRED_VERSION@ or newer.
Simple install procedure