gdk: Fix build on older versions of Gtk+ for the key symbols

Older versions of GTK+/GDK define less keyboard symbols, e.g.
on Debian 6.0 GDK_TouchpadToggle is not defined. Change the
awk script to use #ifdef/#endif for the key symbols.

2013-08-24  Holger Hans Peter Freyther  <holger@freyther.de>

	* mk_enums.awk: Use #ifdef/#endif around key definition.