pgsql: Allow ecpg to be built stand-alone, allow parallel libpq make

Allow ecpg to be built stand-alone, allow parallel libpq make

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/051fd5e0f99b14d7bd76fb800bd077bf394fecd5

Modified Files
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src/interfaces/ecpg/pgtypeslib/Makefile | 1 +
src/interfaces/libpq/Makefile           | 3 +--
2 files changed, 2 insertions(+), 2 deletions(-)