Use std::thread with libc++

boost::thread is only used due to libstdc++ 4.8 missing a bunch of
stuff.

libaegisub/unix/util.cpp

@@ -12,7 +12,13 @@
 // ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 // OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 
+#include <cstddef>
+
+#ifdef _LIBCPP_VERSION
+#include <thread>
+#else
 #include <boost/thread.hpp>
+#endif
 
 namespace agi { namespace util {
 
@@ -25,7 +31,11 @@ timeval time_log() {
 void SetThreadName(const char *) { }
 
 void sleep_for(int ms) {
+#ifdef __clang__
+	std::this_thread::sleep_for(std::chrono::milliseconds(ms));
+#else
 	boost::this_thread::sleep_for(boost::chrono::milliseconds(ms));
+#endif
 }
 
 } }