Backed out revision fb5ab0e91c56, the platform specific messagebox functions don't have the right prototype since they're designed to be used standalone.

src/video/cocoa/SDL_cocoavideo.m

@@ -33,7 +33,6 @@
 #include "SDL_endian.h"
 #include "SDL_cocoavideo.h"
 #include "SDL_cocoashape.h"
-#include "SDL_cocoamessagebox.h"
 #include "SDL_assert.h"
 
 /* Initialization/Query functions */
@@ -135,8 +134,6 @@ Cocoa_CreateDevice(int devindex)
     device->GetClipboardText = Cocoa_GetClipboardText;
     device->HasClipboardText = Cocoa_HasClipboardText;
 
-    device->ShowMessageBox = Cocoa_ShowMessageBox;
-
     device->free = Cocoa_DeleteDevice;
 
     return device;

src/video/uikit/SDL_uikitvideo.m

@@ -35,7 +35,6 @@
 #include "SDL_uikitmodes.h"
 #include "SDL_uikitwindow.h"
 #include "SDL_uikitopengles.h"
-#include "SDL_uikitmessagebox.h"
 
 #define UIKITVID_DRIVER_NAME "uikit"
 
@@ -99,9 +98,6 @@ UIKit_CreateDevice(int devindex)
     device->GL_DeleteContext    = UIKit_GL_DeleteContext;
     device->GL_GetProcAddress   = UIKit_GL_GetProcAddress;
     device->GL_LoadLibrary        = UIKit_GL_LoadLibrary;
-
-    device->ShowMessageBox = UIKit_ShowMessageBox;
-
     device->free = UIKit_DeleteDevice;
 
     device->gl_config.accelerated = 1;

src/video/windows/SDL_windowsvideo.c

@@ -32,7 +32,6 @@
 #include "SDL_windowsvideo.h"
 #include "SDL_windowsframebuffer.h"
 #include "SDL_windowsshape.h"
-#include "SDL_windowsmessagebox.h"
 
 /* Initialization/Query functions */
 static int WIN_VideoInit(_THIS);
@@ -146,8 +145,6 @@ WIN_CreateDevice(int devindex)
     device->GetClipboardText = WIN_GetClipboardText;
     device->HasClipboardText = WIN_HasClipboardText;
 
-    device->ShowMessageBox = WIN_ShowMessageBox;
-
     device->free = WIN_DeleteDevice;
 
     return device;

src/video/x11/SDL_x11video.c

@@ -34,7 +34,6 @@
 #include "SDL_x11shape.h"
 #include "SDL_x11touch.h"
 #include "SDL_x11xinput2.h"
-#include "SDL_x11messagebox.h"
 
 #if SDL_VIDEO_OPENGL_EGL
 #include "SDL_x11opengles.h"
@@ -412,8 +411,6 @@ X11_CreateDevice(int devindex)
     device->GetClipboardText = X11_GetClipboardText;
     device->HasClipboardText = X11_HasClipboardText;
 
-    device->ShowMessageBox = X11_ShowMessageBox;
-
     device->free = X11_DeleteDevice;
 
     return device;