cmake: SDL_process on unix needs vfork symbol

Current FIL-C toolchains don't have vfork

CMakeLists.txt

@@ -3461,7 +3461,10 @@ int main(void)
     return 0;
 }
 " HAVE_POSIX_SPAWN)
-  if(HAVE_POSIX_SPAWN)
+  if(NOT APPLE)
+    check_symbol_exists(vfork "unistd.h" LIBC_HAS_VFORK)
+  endif()
+  if(HAVE_POSIX_SPAWN AND (APPLE OR LIBC_HAS_VFORK))
     sdl_glob_sources(
       "${SDL3_SOURCE_DIR}/src/process/posix/*.c"
       "${SDL3_SOURCE_DIR}/src/process/posix/*.h"