macOS: fix atomics using deprecated functions instead of compiler intrinsics, when SDL is built with the Xcode project.
diff --git a/include/SDL_config_macosx.h b/include/SDL_config_macosx.h
index c494b2e..16d0509 100644
--- a/include/SDL_config_macosx.h
+++ b/include/SDL_config_macosx.h
@@ -130,6 +130,8 @@
#define HAVE_SYSCONF 1
#define HAVE_SYSCTLBYNAME 1
+#define HAVE_GCC_ATOMICS 1
+
/* Enable various audio drivers */
#define SDL_AUDIO_DRIVER_COREAUDIO 1
#define SDL_AUDIO_DRIVER_DISK 1