message: cmake: fix add_subdirectory for Windows
diff: @@ -66,7 +66,7 @@ foreach(plugin ${external_plugins})
  set(header_files "")
  set(impl_header_files "")
  - add_subdirectory(${plugin} ${CMAKE_CURRENT_BINARY_DIR}/${plugin})
  + add_subdirectory(${plugin} ${plugin})
  foreach(source_file ${source_files})
  list(APPEND plugin_source_files "${plugin}/${source_file}")
  endforeach()
  