Fix build with --no-copy-dt-needed-entries ld key

--- configure.in
+++ configure.in
@@ -25,6 +25,7 @@ dnl
 dnl Check for Gtk+ for gapcmon
 dnl
 PKG_CHECK_MODULES(GTK, gtk+-2.0 >= 2.4.0 glib-2.0 gthread-2.0 gconf-2.0)
+GTK_LIBS="$GTK_LIBS -lX11"
 AC_SUBST(GTK_CFLAGS)
 AC_SUBST(GTK_LIBS)