When using the
Duplicate option to create variant dirs,
sometimes you may find files not getting copied to where you
expect (or not at all), or files mysteriously disappearing. These
are usually because of a misconfiguration of some kind in the
SConstruct/SConscript, but they can be tricky to debug. The
--debug=duplicate option shows each time a variant file is
unlinked and relinked from its source (or copied, depending on
settings), and also shows a message for removing "stale"
variant-dir files that no longer have a corresponding source file.
It also prints a line for each target that's removed just before
building, since that can also be mistaken for the same thing.