Fix for boost thread that requires system since 1.50 or so

Hi Kyle,

Solved after trying to compile libnifalcon on OSX 10.8.3.

From http://www.boost.org/doc/libs/1_50_0/doc/html/thread/build.html

Boost.Thread uses by default Boost.System to define the exceptions. For backward compatibility and also for compilers that don't work well with Boost.System the user can define BOOST_THREAD_DONT_USE_SYSTEM .
BOOST_THREAD_USES_SYSTEM is defined when Boost.Thread uses Boost.Move.

Best regards,
Christian