Fix meson.build to work with older meson

Robot suggested this, and it seems to do the trick.

meson.build

@@ -14,11 +14,11 @@ libdng = dependency('libdng', version: '>=1.1.0')
 
 # We only build in support for Wayland/X11 if GTK did so
 optdeps = []
-if gtkdep.get_variable('targets').contains('wayland')
+if dependency('gtk4-wayland', required: false).found()
   optdeps += dependency('gtk4-wayland')
   optdeps += dependency('wayland-client')
 endif
-if gtkdep.get_variable('targets').contains('x11')
+if dependency('x11', required: false).found()
   optdeps += dependency('gtk4-x11')
   optdeps += dependency('x11')
   optdeps += dependency('xrandr')