pgsql: Fix Mkvcbuild.pm builds of test_cloexec.c.

Fix Mkvcbuild.pm builds of test_cloexec.c.

Mkvcbuild.pm scrapes Makefile contents, but couldn't understand the
change made by commit bec2a0aa. Revealed by BF animal hamerkop in
branch REL_16_STABLE.

1. It used += instead of =, which didn't match the pattern that
Mkvcbuild.pm looks for. Drop the +.

2. Mkvcbuild.pm doesn't link PROGRAM executables with libpgport. Apply
a local workaround to REL_16_STABLE only (later branches dropped
Mkvcbuild.pm).

Backpatch-through: 16
Reported-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/175163.1766357334%40sss.pgh.pa.us

Branch
------
REL_18_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/4da5c33a3a046fc81a6b490568801c5739286936

Modified Files
--------------
src/test/modules/test_cloexec/Makefile | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)