Failure building with libgccjit under MacOS Monterey

[Perry E. Metzger]

Howdy! I suspect this is a mistake I'm making somewhere, but I'd like a 
hint on where to look to fix it. When building the Emacs 28 branch on 
the latest MacOS, I unexpectedly get a failure:

  configure: error: The installed libgccjit failed to compile and run a 
test program using
  the libgccjit library; see config.log for the details of the failure.

Looking at the config.log says (I think this is the key line):

  ld: library not found for -lSystem

A few weeks ago I was building JIT versions of Emacs just fine with the 
same configuration; updating the OS might or might not have been the big 
change. Any ideas what I might need to fix?

Perry