sys_time: Support for MSVC. * m4/sys_time_h.m4 (gl_HEADER_SYS_TIME_H_BODY): Invoke gl_PREREQ_SYS_H_WINSOCK2. When testing for 'struct timeval', also include <winsock2.h>. * lib/sys_time.in.h: On MSVC, include <winsock2.h> and hide its function declarations that collide with POSIX. * modules/sys_time (Files): Add m4/sys_socket_h.m4. (Makefile.am): Substitute HAVE_WINSOCK2_H.