message: cmake: don't prepend additional_includes
  If we prepend EXTERNAL_DIR before additional_includes we can't use
  absolute paths in additional_includes.
diff: @@ -104,7 +104,7 @@ include_directories(
  ${external_plugins}
  ${CMAKE_CURRENT_BINARY_DIR}/include
  ${CMAKE_CURRENT_BINARY_DIR}/core
  - SYSTEM ${EXTERNAL_DIR}/${additional_includes}
  + SYSTEM ${additional_includes}
  )
  if (DEFINED additional_includes AND NOT additional_includes STREQUAL "")
  