The stat(2) manpage implies that S_ISLNK is a POSIX macro. In fact, it seems to be a GNU extension. http://www.linuxbase.org/spec/libref.html (and likewise S_ISSOCK) This caused me to develop code on a RedHat machine, which then turned out not to port cleanly to other systems.
Fixed in man-pages-1.31-4, which should make it to Rawhide some day. Thanks for the report, I'll upstream the patch.