message: Throw an error if no component is set for FindCAF
diff: #
  # Use this module as follows:
  #
  -# find_package(CAF)
  +# find_package(CAF [COMPONENTS <core|io|opencl|...>*] [REQUIRED])
  #
  # Variables used by this module (they can change the default behaviour and need
  # to be set before calling find_package):
  # CAF_LIBRARY_$C Library file for component $C
  # CAF_INCLUDE_DIR_$C Include path for component $C
  +if(CAF_FIND_COMPONENTS STREQUAL "")
  + message(FATAL_ERROR "FindCAF requires at least one COMPONENT.")
  +endif()
  +
  # iterate over user-defined components
  foreach (comp ${CAF_FIND_COMPONENTS})
  # we use uppercase letters only for variable names
  