message: Fix custom plugins building in cmake
  The binary dir for out-of-tree libraries needs to be a subdir of whatever contains CMakeCache.txt (i.e. CMAKE_BINARY_DIR) of the project.
  Wasn't always like this. Likely breakage due to cmake update, dunno, don't care.
diff: @@ -92,5 +92,5 @@ endforeach(plugin)
  # Allow specifying out of tree plugins by putting their paths in an env. variable
  foreach(addplugin $ENV{NWNX_ADDITIONAL_PLUGINS})
  - add_subdirectory(${addplugin} ${CMAKE_CURRENT_SOURCE_DIR}/Binaries)
  + add_subdirectory(${addplugin} ${CMAKE_BINARY_DIR}/custom)
  endforeach(addplugin)
  