Link with libX11 explicitly.

--- mail-notification-5.4.dfsg.1.orig/jbsrc/jb.c
+++ mail-notification-5.4.dfsg.1/jbsrc/jb.c
@@ -445,6 +445,8 @@
 
   jb_compile_options_add_libs(object->compile_options, "-lm");
 
+  jb_compile_options_add_libs(object->compile_options, "-lX11");
+
   jb_compile_options_add_package(object->compile_options, "gettext");
   jb_compile_options_add_package(object->compile_options, "gnome");
   jb_compile_options_add_package(object->compile_options, "dbus");