Creating an attribute on UserBuildSettings during SConf breaks
clean/help, which do not run configure tests. Remove that and add a
method to return the value the attribute would have had.
Reported-by: derhass <https://github.com/dxx-rebirth/dxx-rebirth/issues/412>
Fixes: 1be414217c ("Add build time flag to choose whether to enable ADL MIDI support")