VC+d3d12: use __has_include to detect presence of d3d12 ehaders

backport of 4fa50062472795234806e89eccf1fcebbab3407a

include/SDL_config_windows.h

@@ -99,9 +99,11 @@ typedef unsigned int uintptr_t;
 #define HAVE_D3D11_H 1
 #define HAVE_ROAPI_H 1
 #endif
-#if defined(WDK_NTDDI_VERSION) && WDK_NTDDI_VERSION > 0x0A000008 /* 10.0.19041.0 */
+#if defined(__has_include)
+#if __has_include(<d3d12.h>) && __has_include(<d3d12sdklayers.h>)
 #define HAVE_D3D12_H 1
 #endif
+#endif
 #if defined(_WIN32_MAXVER) && _WIN32_MAXVER >= 0x0603  /* Windows 8.1 SDK */
 #define HAVE_SHELLSCALINGAPI_H 1
 #endif