Add "-lrt" in Makefile for Solaris

On Solaris, to use nanosleep must link the rt libraray. Below is the manual of nanosleep on Solaris:

Realtime Library Functions                         nanosleep(3RT)

NAME
     nanosleep - high resolution sleep

SYNOPSIS
     cc [ flag... ] file... -lrt [ library... ]
     #include <time.h>

     int nanosleep(const struct timespec *rqtp,  struct  timespec *rmtp);