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@sss.pgh.pa.us>
Discussion: https://postgr.es/m/175163.1766357334%40sss.pgh.pa.us