libtorrent: Switch to static library.

libtorrent is only used by rtorrent. Switch to a static library to save
space.

Removed libsigc++ dependency. It seems it's not needed.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

libs/libtorrent/patches/010-usleep.patch

@@ -0,0 +1,54 @@
+--- a/src/data/hash_queue.cc
++++ b/src/data/hash_queue.cc
+@@ -38,9 +38,10 @@
+ 
+ #define __STDC_FORMAT_MACROS
+ 
++#include <chrono>
++#include <thread>
+ #include <functional>
+ #include <rak/functional.h>
+-#include <unistd.h>
+ 
+ #include "torrent/exceptions.h"
+ #include "torrent/data/download_data.h"
+@@ -137,7 +138,7 @@ HashQueue::remove(HashQueueNode::id_type id) {
+ 
+       while ((done_itr = m_done_chunks.find(hash_chunk)) == m_done_chunks.end()) {
+         pthread_mutex_unlock(&m_done_chunks_lock);
+-        usleep(100);
++        std::this_thread::sleep_for(std::chrono::microseconds(100));
+         pthread_mutex_lock(&m_done_chunks_lock);
+       }
+ 
+--- a/src/torrent/utils/thread_base.cc
++++ b/src/torrent/utils/thread_base.cc
+@@ -37,8 +37,9 @@
+ #include "config.h"
+ 
+ #include <cstring>
++#include <chrono>
++#include <thread>
+ #include <signal.h>
+-#include <unistd.h>
+ 
+ #include "exceptions.h"
+ #include "poll.h"
+@@ -97,7 +98,7 @@ thread_base::stop_thread_wait() {
+   release_global_lock();
+ 
+   while (!is_inactive()) {
+-    usleep(1000);
++    std::this_thread::sleep_for(std::chrono::milliseconds(1));
+   }  
+ 
+   acquire_global_lock();
+@@ -161,7 +162,7 @@ thread_base::event_loop(thread_base* thread) {
+       }
+ 
+       // Add the sleep call when testing interrupts, etc.
+-      // usleep(50);
++      // std::this_thread::sleep_for(std::chrono::microseconds(50));
+ 
+       int poll_flags = 0;
+