Check for whether librt is needed for clock_gettime.

Author: kareem-wolfssl

configure.ac

@@ -728,6 +728,11 @@ then
     AM_CFLAGS="$AM_CFLAGS -DNO_DEV_RANDOM -DNO_WRITEV -DNO_STDIO_FILESYSTEM -DWOLFSSL_NO_SOCK -DWOLFSSL_USER_IO"
 fi
 
+if test "$ENABLED_RNG" = "yes" || test "$ENABLED_BENCHMARK" = "yes"
+then
+    AC_SEARCH_LIBS([clock_gettime],[rt])
+fi
+
 
 # MATH LIBRARY SELECTION