C99 + GCC extensions + POSIX:

  #define for_duration(seconds, body)                     \
    {                                                     \
      pthread_t tid_task, tid_watcher;                    \
                                                          \
      void* task(void* arg) {                             \
        body ;                                            \
        pthread_cancel(tid_watcher);                      \
        return NULL;                                      \
      }                                                   \
                                                          \
      void* watcher(void* arg) {                          \
        sleep((seconds));                                 \
        pthread_cancel(tid_task);                         \
        return NULL;                                      \
      }                                                   \
                                                          \
      pthread_create(&tid_task, NULL, &task, NULL);       \
      pthread_create(&tid_watcher, NULL, &watcher, NULL); \
      pthread_join(tid_task, NULL);                       \
    }                             

Use as:

  for_duration(10, {
      while (true) {
        printf("Infinite loop! where is your Godel now?\n");
      }
    });
  

Or with single brace-less statements:

  for_duration(10, while (true) printf("something\n") );
  

Lexical scope is sane:

  int i = 0;
  for_duration(10, {
      while (true) {
        printf("%d\n", i++);
      }
    });

My comparison isn't quite fair, because your threading library provides 'with-timeout', while pthreads doesn't. If you factored this out, say with a signature like:

  void with_timeout(unsigned int seconds, void (*func)(void));

(in analogy to the common lisp function), then the macro part becomes just four lines:

  #define for_duration(seconds, body) {			\
    void func() { body ; }				\
    with_timeout((seconds), &func);			\
  }

I acknowledge that the lisp solution is rather more elegant. Also, my C macro is potentially dangerous because it is unhygenic. (It shadows outer declarations of 'task()', 'watcher()', 'tid_task', 'tid_watcher', and 'arg', in the body of 'body'.)