SDL_spinlock.c: define HAVE_GCC_ATOMICS for windows/clang builds,

... if not already defined.
Fixes https://github.com/libsdl-org/SDL/issues/5333

diff --git a/src/atomic/SDL_spinlock.c b/src/atomic/SDL_spinlock.c
index bdd347e..4f10741 100644
--- a/src/atomic/SDL_spinlock.c
+++ b/src/atomic/SDL_spinlock.c
@@ -28,6 +28,12 @@
 #include "SDL_mutex.h"
 #include "SDL_timer.h"

+#if defined(__WIN32__) && defined(__clang__)
+# ifndef HAVE_GCC_ATOMICS
+# define HAVE_GCC_ATOMICS 1
+# endif
+#endif
+
 #if !defined(HAVE_GCC_ATOMICS) && defined(__SOLARIS__)
 #include <atomic.h>
 #endif

src/atomic/SDL_spinlock.c

@@ -28,6 +28,12 @@
 #include "SDL_mutex.h"
 #include "SDL_timer.h"
 
+#if defined(__WIN32__) && defined(__clang__)
+# ifndef HAVE_GCC_ATOMICS
+# define HAVE_GCC_ATOMICS 1
+# endif
+#endif
+
 #if !defined(HAVE_GCC_ATOMICS) && defined(__SOLARIS__)
 #include <atomic.h>
 #endif