message: Fix: allow compiling AliRoot without AliEn
  This used to be possible but was broken when AliEn-ROOT-Legacy
  was extracted from ROOT. When AliEn is missing, just print a
  warning instead of throwin a fatal error.
diff: @@ -210,7 +210,7 @@ if(ROOTSYS)
  set(ROOT_HASALIEN TRUE)
  else(ALIEN)
  - message(FATAL_ERROR "No AliEn installation found. Please set \"ALIEN\" to point to your AliEn installation.")
  + message(WARNING "No AliEn installation found. Please set \"ALIEN\" to point to your AliEn installation.")
  endif(ALIEN)
  # Checking for xml support
  