Fix builds with gtk_layer_shell=false or omitted.

Fixes #3366

Author: mtwebster

meson.build

@@ -139,8 +139,8 @@ conf.set('HAVE_SELINUX', libselinux_enabled)
 gtk_layer_shell_enabled = get_option('gtk_layer_shell')
 if gtk_layer_shell_enabled
   gtk_layer_shell = dependency('gtk-layer-shell-0', version: '>=0.8', required: false)
+  conf.set('HAVE_GTK_LAYER_SHELL', gtk_layer_shell.found())
 endif
-conf.set('HAVE_GTK_LAYER_SHELL', gtk_layer_shell.found())
 
 # make sure pango development files are installed
 pango = dependency('pango', version: '>=1.40.0')