cmake: fix uses of undefined macro identifiers (-Wundef)

include/SDL_platform.h

@@ -166,6 +166,12 @@
 #define WINAPI_FAMILY_WINRT 0
 #endif /* HAVE_WINAPIFAMILY_H */
 
+#if (HAVE_WINAPIFAMILY_H) && defined(WINAPI_FAMILY_PHONE_APP)
+#define SDL_WINAPI_FAMILY_PHONE (WINAPI_FAMILY == WINAPI_FAMILY_PHONE_APP)
+#else
+#define SDL_WINAPI_FAMILY_PHONE 0
+#endif
+
 #if WINAPI_FAMILY_WINRT
 #undef __WINRT__
 #define __WINRT__ 1