message: Fix handling missing X11 libraries (on macos)
  when missing, throw a warning and don't build ZMQhistViewer
diff: @@ -107,11 +107,13 @@ target_link_libraries(${MODULE} ${LIBDEPS})
  set_target_properties(${MODULE} PROPERTIES COMPILE_FLAGS "")
  # System dependent: Modify the way the library is build
  -if(${CMAKE_SYSTEM} MATCHES Darwin)
  +if(${CMAKE_SYSTEM} MATCHES Darwin AND X11_FOUND)
  set_target_properties(${MODULE} PROPERTIES LINK_FLAGS "-undefined dynamic_lookup")
  include_directories(${X11_INCLUDE_DIR})
  SET(CMAKE_EXE_LINKER_FLAGS
  "${CMAKE_EXE_LINKER_FLAGS} -L${X11_LIB_DIR}")
  +else()
  + message(WARNING "X11 libraries not found - will NOT build ZMQhistViewer")
  endif(${CMAKE_SYSTEM} MATCHES Darwin)
  # Installation
  @@ -134,8 +136,10 @@ if(ZEROMQ_FOUND)
  target_link_libraries(ZMQROOTmerger HLTbase AliHLTGlobal AliZMQhelpers RAWDatabase Geom Graf MathCore Net Tree EG Gpad Matrix Minuit Physics VMC Thread STEERBase XMLParser Graf3d RIO Hist Core pthread)
  add_executable(ZMQhistSource ZMQhistSource.cxx)
  target_link_libraries(ZMQhistSource HLTbase AliHLTGlobal AliZMQhelpers RAWDatabase Geom Graf MathCore Net Tree EG Gpad Matrix Minuit Physics VMC Thread STEERBase XMLParser Graf3d RIO Hist Core)
  + if (X11_FOUND)
  add_executable(ZMQhistViewer ZMQhistViewer.cxx)
  target_link_libraries(ZMQhistViewer HLTbase AliHLTGlobal AliZMQhelpers AliHLTUtil RAWDatabase Geom Graf MathCore Net Tree EG Gpad Matrix Minuit Physics VMC Thread STEERBase XMLParser Graf3d RIO Hist Core Hist Rint X11)
  + endif (X11_FOUND)
  add_executable(ZMQproxy ZMQproxy.cxx)
  target_link_libraries(ZMQproxy HLTbase AliHLTGlobal AliZMQhelpers RAWDatabase Geom Graf MathCore Net Tree EG Gpad Matrix Minuit Physics VMC Thread STEERBase XMLParser Graf3d RIO Hist Core)
  add_executable(ZMQHLTchain ZMQHLTchain.cxx)
  