Use `SDL_NORETURN`

I misread SDL's code and thought that SDL's `begin_code.h` was internal
only to SDL. It turns out you get it when you include basically any
header, such as `SDL_stdinc.h`. So use it directly instead of copying it
for our own.

desktop_version/src/Exit.h

@@ -1,14 +1,8 @@
 #ifndef VVV_EXIT_H
 #define VVV_EXIT_H
 
-#ifdef __GNUC__
-#   define VVV_NORETURN __attribute__((noreturn))
-#elif defined(_MSC_VER)
-#   define VVV_NORETURN __declspec(noreturn)
-#else
-#   define VVV_NORETURN
-#endif
+#include <SDL_stdinc.h>
 
-VVV_NORETURN void VVV_exit(const int exit_code);
+SDL_NORETURN void VVV_exit(const int exit_code);
 
 #endif /* VVV_EXIT_H */

desktop_version/src/main.cpp

@@ -708,7 +708,7 @@ static void cleanup(void)
     FILESYSTEM_deinit();
 }
 
-VVV_NORETURN void VVV_exit(const int exit_code)
+SDL_NORETURN void VVV_exit(const int exit_code)
 {
     cleanup();
     exit(exit_code);