Update Makefile

* Added -lX11 to GTK2 X_LIBS to solve link symbol problem
  reported as FreeBSD PR ports/184913
  <http://lists.freebsd.org/pipermail/freebsd-ports-bugs/2013-December/268151.html>
  Hint at: <http://lists.freebsd.org/pipermail/freebsd-ports-bugs/2013-December/268174.html>