Is anyone willing to help me a bit to come up with a pull request?

I took some ideas from here:
https://fossies.org/diffs/xscreensaver/5.30_vs_5.32/utils/thread_util.c-diff.html

Citation from that patch:
   Clang 3.0 has a partial implementation of GNU atomics; 3.1 rounds it out.

   http://llvm.org/viewvc/llvm-project/cfe/tags/RELEASE_30/final/include/clang/Basic/Builtins.def?view=markup
   http://llvm.org/viewvc/llvm-project/cfe/tags/RELEASE_31/final/include/clang/Basic/Builtins.def?view=markup

   Apple changes the Clang version to track Xcode versions; use
   __apple_build_version__ to distinguish between the two.

   Xcode 4.3 uses Apple LLVM 3.1, which corresponds to Clang 3.1.
   https://en.wikipedia.org/wiki/Xcode

One should probably adapt this into "at least clang 3.3 (or perhaps even 3.4?) is needed". Now, the basic test in configure.ac is the following:

    volatile int val = 1;
    int main() {
        __atomic_load_n(&val, __ATOMIC_SEQ_CST);
        return 0;
    }

With clang on 10.7 (Apple LLVM version 4.2 (clang-425.0.28) (based on LLVM 3.2svn)) this compiles, runs and returns 0. Evidently a more complex test is needed to trigger the error, but I'm not sure that that test should be since I don't have enough experience with "atomic". If you have a clue what functionality I should test, I will gladly do it and try to come up with a better test.

An alternative could be to blacklist a compiler and this is what I am proposing here as a very naive approach:

    volatile int val = 1;
    int main() {
 
        __atomic_load_n(&val, __ATOMIC_SEQ_CST);

    #define VERSION_CHECK(cc_major, cc_minor, req_major, req_minor) \
        ((cc_major) > (req_major) || \
        (cc_major) == (req_major) && (cc_minor) >= (req_minor))

    #if defined(__clang__)
        #if defined(__apple_build_version__)
            // either one test or the other should work
            // #if __apple_build_version__ < 5000000
            #if !VERSION_CHECK(__clang_major__, __clang_minor__, 5, 0)
                this_should_fail();
            #endif
        // not sure if this is 3.3 or 3.4
        #elif !VERSION_CHECK(__clang_major__, __clang_minor__, 3, 3)
            this_should_fail();
        #endif
    #endif

      return 0;
    }