Remove traces of Sun -lposix4.

This was a library on ancient Solaris systems, which was eventually
replaced by -lrt, itself now redundant on that OS.

Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Reviewed-by: Tristan Partin <tristan@neon.tech>
Discussion: https://postgr.es/m/CA%2BhUKGLLVx4drdvXats9PxH3eeB%2BE2NkJReJ%2BRGmaOpU%3D9rgEA%40mail.gmail.com

configure

@@ -12163,7 +12163,7 @@ return clock_gettime ();
   return 0;
 }
 _ACEOF
-for ac_lib in '' rt posix4; do
+for ac_lib in '' rt; do
   if test -z "$ac_lib"; then
     ac_res="none required"
   else