Good point. I have included the guts of your little test program (modified to do polling) into the existing autoconf test program that decides on the #define USE_AIO_ATOMIC_BUILTIN_COMP_SWAP. See config/c-library.m4. I hope this goes some way to answer your concern about robustness, as at least now if the implementation changes in some way that renders the polling ineffective, it will be caught in configure.
I meant to add that by including this test, which involves a fork(), in the autoconf tester, on Windows USE_AIO_ATOMIC_BUILTIN_COMP_SWAP would always by un-defined. (But could then be defined manually if someone wanted to give it a try)
