Use _SDL_HAS_BUILTIN

I think this is because if you both check that __has_builtin is defined
and use it in the same 'if' preprocessor statement, it can error because
there's no equivalent to short-circuiting in preprocessor statements.
_SDL_HAS_BUILTIN should be safer.

desktop_version/src/Unreachable.h

@@ -7,7 +7,7 @@ SDL_NORETURN SDL_INLINE void VVV_unreachable(void)
 {
     /* __builtin_unreachable() and __assume(0) execute undefined behavior.
      * Otherwise, a noreturn function returning is also undefined behavior. */
-#if defined(__has_builtin) && __has_builtin(__builtin_unreachable)
+#if _SDL_HAS_BUILTIN(__builtin_unreachable)
     __builtin_unreachable();
 #elif defined(_MSC_VER)
     __assume(0);