SDL: PreseedDOSCache.cmake: Add vfork check.

SDL Commits
1 
From 18b29bdd588b970008327fae3d1240d66fa57366 Mon Sep 17 00:00:00 2001
From: "Ryan C. Gordon" <[EMAIL REDACTED]>
Date: Tue, 2 Jun 2026 11:24:34 -0400
Subject: [PATCH] PreseedDOSCache.cmake: Add vfork check.

(djgpp actually has this symbol, believe it or not. Not sure what it could
possibly do, but it says it has it!)
---
 cmake/PreseedDOSCache.cmake | 1 +
 1 file changed, 1 insertion(+)

diff --git a/cmake/PreseedDOSCache.cmake b/cmake/PreseedDOSCache.cmake
index 6ca5fd53bf7cd..1b2a70aab41bd 100644
--- a/cmake/PreseedDOSCache.cmake
+++ b/cmake/PreseedDOSCache.cmake
@@ -141,6 +141,7 @@ if(CMAKE_SYSTEM_NAME STREQUAL "DOS")
       set(LIBC_HAS_TRUNC                                 "1"   CACHE INTERNAL "Have symbol trunc")
       set(LIBC_HAS_TRUNCF                                "1"   CACHE INTERNAL "Have symbol truncf")
       set(LIBC_HAS_UNSETENV                              "1"   CACHE INTERNAL "Have symbol unsetenv")
+      set(LIBC_HAS_VFORK                                 "1"   CACHE INTERNAL "Have symbol vfork")
       set(LIBC_HAS_VSNPRINTF                             "1"   CACHE INTERNAL "Have symbol vsnprintf")
       set(LIBC_HAS_VSSCANF                               "1"   CACHE INTERNAL "Have symbol vsscanf")
       set(LIBC_HAS_WCHAR_H                               "1"   CACHE INTERNAL "Have include wchar.h")