But this is what I don't understand. When was there someone with authority to force all the Unices to adopt (1)? Autoconf came out in 1991, so clearly the problem existed at that point. That's coincidentally also the year Linux was first released, so it can't be blamed.

It seems to me that Unix was never a cathedral. Each specific operating system may have been one, but they clearly weren't more compatible back in 1991 than a Protestant and a Catholic cathedral are, since someone felt the need to write autoconf.

Please give me an example of this shining 20-th century "cathedral development", because I don't see one.