Fix -Wundef warnings due to use of unguarded SDL_ATOMIC_DISABLED

src/atomic/SDL_spinlock.c

@@ -60,7 +60,7 @@ extern __inline int _SDL_xchg_watcom(volatile int *a, int v);
 SDL_bool
 SDL_AtomicTryLock(SDL_SpinLock *lock)
 {
-#if SDL_ATOMIC_DISABLED
+#if defined(SDL_ATOMIC_DISABLED)
     /* Terrible terrible damage */
     static SDL_mutex *_spinlock_mutex;
 

src/dynapi/SDL_dynapi.c

@@ -484,7 +484,7 @@ static void SDL_InitDynamicAPI(void)
 /* SDL_AtomicLock calls SDL mutex functions to emulate if
    SDL_ATOMIC_DISABLED, which we can't do here, so in such a
    configuration, you're on your own. */
-#if !SDL_ATOMIC_DISABLED
+#ifndef SDL_ATOMIC_DISABLED
     static SDL_SpinLock lock = 0;
     SDL_AtomicLock_REAL(&lock);
 #endif
@@ -494,7 +494,7 @@ static void SDL_InitDynamicAPI(void)
         already_initialized = SDL_TRUE;
     }
 
-#if !SDL_ATOMIC_DISABLED
+#ifndef SDL_ATOMIC_DISABLED
     SDL_AtomicUnlock_REAL(&lock);
 #endif
 }