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);

src/Makefile

@@ -58,7 +58,7 @@ ifeq ($(uname_S),SunOS)
 	# SunOS
 	INSTALL=cp -pf
 	FINAL_CFLAGS+= -D__EXTENSIONS__ -D_XPG6
-	FINAL_LIBS+= -ldl -lnsl -lsocket -lpthread
+	FINAL_LIBS+= -ldl -lnsl -lsocket -lpthread -lrt
 else
 ifeq ($(uname_S),Darwin)
 	# Darwin (nothing to do)