Make feature test check for locking

tests/features/S2N_ATOMIC_SUPPORTED.c

@@ -14,10 +14,17 @@
  */
 
 #include <signal.h>
+#include <stddef.h>
 
 int main() {
+    /* Atomic builtins are supported by gcc 4.7.3 and later. */
     sig_atomic_t atomic = 0;
     __atomic_test_and_set(&atomic, __ATOMIC_RELAXED);
     __atomic_clear(&atomic, __ATOMIC_RELAXED);
+
+    /* _Static_assert is supported for C99 by gcc 4.6 and later,
+     * so using it here shouldn't limit use of the atomic builtins. */
+    _Static_assert(__atomic_always_lock_free(sizeof(sig_atomic_t), NULL),
+            "Atomic operations in this environment would require locking");
     return 0;
 }