A tiny nit; the new-build claims that old style ALT_ environment variables are deprecated (true), ought to be unset (true), and are ignored (apparently not true).

I was having problems building with debug level slowdebug and fastdebug on MacOS; on a whim, I unset all my ALT variables, and the problems (in particular, the problem of complaining about no rule for universal_debug) went away. So I think that ALT_MACOSX_UNIVERSAL needs to be more effectively ignored -- or less effectively ignored, the build could just refuse to proceed if any are set.