Fix external console bug

This commit fixes a bug with gnome-terminal in conjunction with the external console option. Gnome have removed the --disable-factory argument and as such, gnome-terminal appears to be unusable with Monodevelop, this will make it so xterm is used on Gnome desktops which does work. Refer to https://bugzilla.xamarin.com/show_bug.cgi?id=12684 for more detail.