HAVE_WORKING_LINK is meant to indicate support for hard links, mainly
for Windows.  Here it is used for soft links on Unix, and the
functionality is optional anyway, so we can just make it error out
normally if needed.

Discussion: https://www.postgresql.org/message-id/flat/72fff73f-dc9c-4ef4-83e8-d2e60c98df48%402ndquadrant.com