Link with boost_chrono library on all platforms

Some versions of the boost thread library need the boost chrono library to
avoid linker errors like:

Undefined symbols:
  "boost::chrono::steady_clock::now()", referenced from:
      boost::cv_status boost::condition_variable::wait_for<long long, boost::ratio<1ll, 1000000000ll> >(boost::unique_lock<boost::mutex>&, boost::chrono::duration<long long, boost::ratio<1ll, 1000000000ll> > const&)in bitcoinrpc.o

The windows builds already linked with boost_chrono and the OSX 10.5-compatible build was failing. This pull makes the boost libraries we link against consistent on Windows/OSX/Linux.