haiku: Implement message box for Haiku

Add implementation for functions:

SDL_ShowSimpleMessageBox()
SDL_ShowMessageBox()

Add simple customization support also.
Fix build for x86_gcc2.

Partially fixes Bugzilla #4442.

src/main/haiku/SDL_BeApp.cc

@@ -48,13 +48,14 @@ extern "C" {
 static int SDL_BeAppActive = 0;
 static SDL_Thread *SDL_AppThread = NULL;
 
+/* Default application signature */
+const char *signature = "application/x-SDL-executable";
+
 static int
 StartBeApp(void *unused)
 {
     BApplication *App;
 
-    // default application signature
-    const char *signature = "application/x-SDL-executable";
     // dig resources for correct signature
     image_info info;
     int32 cookie = 0;

src/main/haiku/SDL_BeApp.h

@@ -31,6 +31,9 @@ extern int SDL_InitBeApp(void);
 /* Quit the Be Application, if there's nothing left to do */
 extern void SDL_QuitBeApp(void);
 
+/* Be Application Signature*/
+extern const char *signature;
+
 /* vi: set ts=4 sw=4 expandtab: */
 
 #ifdef __cplusplus