inet_ntop, inet_pton: Avoid conflict with native Windows functions. * m4/sys_socket_h.m4 (gl_PREREQ_SYS_H_WS2TCPIP): New macro, extracted from gl_PREREQ_SYS_H_SOCKET. (gl_PREREQ_SYS_H_SOCKET): Invoke it. * m4/arpa_inet_h.m4 (gl_HEADER_ARPA_INET): Invoke gl_PREREQ_SYS_H_WS2TCPIP. * modules/arpa_inet (Files): Add m4/sys_socket_h.m4, m4/socklen.m4. (Makefile.am): Substitute HAVE_WS2TCPIP_H. * lib/arpa_inet.in.h: Include <ws2tcpip.h>.