Add port/strnlen support to libpq and ecpg Makefiles.

Tom Lane <tgl@sss.pgh.pa.us>

Commit: 46912d9b1504cfaede1b22811039028a75f76ab8
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2017-10-11T15:28:04Z
Releases: 11.0
Add port/strnlen support to libpq and ecpg Makefiles.

In the wake of fffd651e8, any makefile that pulls in snprintf.c
from src/port/ needs to be prepared to pull in strnlen.c as well.
Per buildfarm.

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