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

This reverts commit fe5468799b0a7cd8fbcc42db1328dea9a80420ce.

cmake/sdlchecks.cmake

@@ -459,7 +459,10 @@ 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 AND HAVE_XEXT_H)
+    if(X11_LIB)
+      if(NOT HAVE_XEXT_H)
+        message_error("Missing Xext.h, maybe you need to install the libxext-dev package?")
+      endif()
 
       set(HAVE_X11 TRUE)
       set(HAVE_SDL_VIDEO TRUE)