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: /messages/by-id/E5Dc85EGUY4wyG8cjAU0qoEdCJxGK_qhW1s9qSuYq9A@mail.gmail.com

Author: Dagfinn Ilmari Mannsåker (for libpq)

Backpatch-through: 12

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/297a174d25b80450aa2bdb058d0a5f2e65f3ba37

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