cmake: consider X11 as not available when xext.h is not found

This is user friendlier than straight up erroring

Backport of 16ad3e5af6ac12af6d6e06c49d78f4d6ec081519

cmake/sdlchecks.cmake

@@ -459,10 +459,7 @@ macro(CheckX11)
     find_file(HAVE_XDBE_H NAMES "X11/extensions/Xdbe.h" HINTS "${X_INCLUDEDIR}")
     find_file(HAVE_XEXT_H NAMES "X11/extensions/Xext.h" HINTS "${X_INCLUDEDIR}")
 
-    if(X11_LIB)
-      if(NOT HAVE_XEXT_H)
-        message_error("Missing Xext.h, maybe you need to install the libxext-dev package?")
-      endif()
+    if(X11_LIB AND HAVE_XEXT_H)
 
       set(HAVE_X11 TRUE)
       set(HAVE_SDL_VIDEO TRUE)