Hm ... I just gave it some more thought and think that I might be wrong: At some places rustc/rustpkg might use the absolute path and this would then result in strange results since it would then depend on the current working dir to find the correct files.

Anyway I don't think a check here should be needed, because other autotools software doesn't check either AFAIK. Also there aren't any checks for CFG_PREFIX either and setting this one to a relative path would mess up even more.