Use a timer to avoid polling

The timer will be fired asynchronously to handle
time management flags, while other threads are
searching.

This implementation uses a thread waiting on a
timed condition variable instead of real timers.
This approach allow to reduce platform dependant
code to a minimum and also is the most portable given
that timers libraries are very different among platforms
and also the best ones are not compatible with olds
Windows.

Also retire the now unused polling code.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/search.h

@@ -67,5 +67,7 @@ struct SearchLimits {
 extern void init_search();
 extern int64_t perft(Position& pos, Depth depth);
 extern bool think(Position& pos, const SearchLimits& limits, Move searchMoves[]);
+extern void do_uci_async_cmd(const std::string& cmd);
+extern void do_timer_event();
 
 #endif // !defined(SEARCH_H_INCLUDED)