Check if -lrt is required for nanosleep, as is the case on Solaris.

inspired by a patch from Fabian Groffen, grobian gentoo org

Originally committed as revision 17537 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -1900,6 +1900,9 @@ elif check_func dlopen -ldl; then
     ldl=-ldl
 fi
 
+# Solaris has nanosleep in -lrt, OpenSolaris no longer needs that
+check_func nanosleep || { check_func nanosleep -lrt && add_extralibs -lrt; }
+
 check_func  fork
 check_func  gethrtime
 check_func  getrusage