can not support recursively adding tasks

[etorth]

following code get stuck.

#include <CTPL/ctpl_stl.h>
#include <iostream>

ctpl::thread_pool p(10);

long f(int, long n)
{
    if(n == 0) return 0;
    if(n == 1) return 1;

    auto f1 = p.push(f, n - 1);
    auto f2 = p.push(f, n - 2);

    return f1.get() + f2.get();
}

int main()
{
    std::cout << f(0, 7);
    return 10;
}

This makes it's hard for a global thread pool.