message: Remove FATAL condition in case ALIEN is not set
  FATAL condition makes it impossible to compile AliPhysics
  with the o2 defaults which deactivate alien. Configuration
  of ALIEN should be handled by the build recipe.
diff: @@ -206,7 +206,8 @@ if(ROOTSYS)
  set(ROOT_HASALIEN TRUE)
  else(ALIEN)
  - message(FATAL_ERROR "ROOT was built with AliEn support but no AliEn installation found. Please set \"ALIEN\" to point to your AliEn installation.")
  + set(ROOT_HASALIEN FALSE)
  + message(WARNING "ROOT was built with AliEn support but no AliEn installation found. Please set \"ALIEN\" to point to your AliEn installation.")
  endif(ALIEN)
  # Checking for xml support
  