Link against boost_chrono on all platforms

Currently, Windows and OS X builds link against boost_chrono, but *nix builds do not.
While this works fine on most Linux systems, including gitian, on some other systems linking against boost_chrono is necessary.
I noticed this when I tried to compile on my Gentoo box with boost 1.52 and got:
`undefined reference to 'boost::chrono::system_clock::now()'`