Move an autoconfig file into config/, get rid of a no-longer-used file.

Follow-up for #517.

config/ax_lib_socket_nsl.m4

@@ -1,40 +0,0 @@
-# ===========================================================================
-#     http://www.gnu.org/software/autoconf-archive/ax_lib_socket_nsl.html
-# ===========================================================================
-#
-# SYNOPSIS
-#
-#   AX_LIB_SOCKET_NSL
-#
-# DESCRIPTION
-#
-#   This macro figures out what libraries are required on this platform to
-#   link sockets programs.
-#
-#   The common cases are not to need any extra libraries, or to need
-#   -lsocket and -lnsl. We need to avoid linking with libnsl unless we need
-#   it, though, since on some OSes where it isn't necessary it will totally
-#   break networking. Unisys also includes gethostbyname() in libsocket but
-#   needs libnsl for socket().
-#
-# LICENSE
-#
-#   Copyright (c) 2008 Russ Allbery <rra@stanford.edu>
-#   Copyright (c) 2008 Stepan Kasal <kasal@ucw.cz>
-#   Copyright (c) 2008 Warren Young <warren@etr-usa.com>
-#
-#   Copying and distribution of this file, with or without modification, are
-#   permitted in any medium without royalty provided the copyright notice
-#   and this notice are preserved. This file is offered as-is, without any
-#   warranty.
-
-#serial 6
-
-AU_ALIAS([LIB_SOCKET_NSL], [AX_LIB_SOCKET_NSL])
-AC_DEFUN([AX_LIB_SOCKET_NSL],
-[
-	AC_SEARCH_LIBS([gethostbyname], [nsl])
-	AC_SEARCH_LIBS([socket], [socket], [], [
-		AC_CHECK_LIB([socket], [socket], [LIBS="-lsocket -lnsl $LIBS"],
-		[], [-lnsl])])
-])

configure.ac

@@ -25,7 +25,7 @@
 
 # Initialize the autoconf system for the specified tool, version and mailing list
 AC_INIT(iperf, 3-CURRENT, https://github.com/esnet/iperf, iperf, http://software.es.net/iperf/)
-m4_include([ax_check_openssl.m4])
+m4_include([config/ax_check_openssl.m4])
 AC_LANG(C)
 
 # Specify where the auxiliary files created by configure should go. The config