SDL: Allow overriding SDL_FORK_MESSAGEBOX in build environment (12b56)

SDL Commits
1 
From 12b56f5447c20c93fd5b8867da6851aa4ab15eeb Mon Sep 17 00:00:00 2001
From: Caleb Heuer <[EMAIL REDACTED]>
Date: Thu, 26 Jun 2025 11:37:19 -0600
Subject: [PATCH] Allow overriding SDL_FORK_MESSAGEBOX in build environment

(cherry picked from commit cd98b6611485f801fc9d5ccd59dddf273a468bc8)
---
 src/video/x11/SDL_x11messagebox.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/src/video/x11/SDL_x11messagebox.c b/src/video/x11/SDL_x11messagebox.c
index ab2174e02a0fd..cbc3c98c18c04 100644
--- a/src/video/x11/SDL_x11messagebox.c
+++ b/src/video/x11/SDL_x11messagebox.c
@@ -30,7 +30,10 @@
 #include <X11/keysym.h>
 #include <locale.h>
 
+#ifndef SDL_FORK_MESSAGEBOX
 #define SDL_FORK_MESSAGEBOX 1
+#endif
+
 #define SDL_SET_LOCALE      1
 
 #if SDL_FORK_MESSAGEBOX