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

src/core/linux/SDL_fcitx.c

@@ -28,7 +28,7 @@
 #include "../../events/SDL_keyboard_c.h"
 #include "SDL_dbus.h"
 #include "SDL_syswm.h"
-#if SDL_VIDEO_DRIVER_X11
+#ifdef SDL_VIDEO_DRIVER_X11
 #  include "../../video/x11/SDL_x11video.h"
 #endif
 #include "SDL_hints.h"
@@ -449,7 +449,7 @@ void SDL_Fcitx_UpdateTextRect(const SDL_Rect *rect)
 
     SDL_GetWindowPosition(focused_win, &x, &y);
 
-#if SDL_VIDEO_DRIVER_X11
+#ifdef SDL_VIDEO_DRIVER_X11
     if (info.subsystem == SDL_SYSWM_X11) {
         SDL_DisplayData *displaydata = (SDL_DisplayData *) SDL_GetDisplayForWindow(focused_win)->driverdata;