This change defines SHLIB_PREREQS for the libpgport dependency, rather
than using a makefile rule.  This was broken in PG 12.

Reported-by: Filip Janus

Discussion: https://postgr.es/m/E5Dc85EGUY4wyG8cjAU0qoEdCJxGK_qhW1s9qSuYq9A@mail.gmail.com

Author: Dagfinn Ilmari Mannsåker (for libpq)

Backpatch-through: 12