link: fix platform bugs * m4/link.m4 (gl_FUNC_LINK): Detect Solaris and Cygwin bugs. * lib/link.c (link): Work around them. Fix related mingw bug. * m4/unistd_h.m4 (gl_UNISTD_H_DEFAULTS): Add REPLACE_LINK. * modules/unistd (Makefile.am): Substitute it. * lib/unistd.in.h (link): Declare replacement. * doc/posix-functions/link.texi (link): Document this. * modules/link (Depends-on): Add strdup-posix, sys_stat. Signed-off-by: Eric Blake <ebb9@byu.net>