So, I discovered that if you have your $PATH set so a ld different from

GNU ld comes first, checking the usage of collect2 gives that instead of
GNU ld, even if GNU ld would be the one that would get used if we link using
gcc.  It's much better, apparently, to ask gcc directly what the path to
GNU ld is (provided it's there at all and gcc knows about it), and ask
the result if it's a GNU or not.  The bonus is that our GNU ld detection
mechanism got shorter and easier to understand...