Fix link failures due to snprintf/strerror changes.

Fix link failures due to snprintf/strerror changes.

snprintf.c requires isnan(), which requires -lm on some platforms.
libpq never bothered with -lm before, but now it needs it.

strerror.c tries to translate a string or two, which requires -lintl.
We'd managed never to need that anywhere in ecpg/pgtypeslib/ before,
but now we do.

Per buildfarm and a report from Peter Eisentraut.

Discussion: https://postgr.es/m/20180926190934.ea4xvzhkayuw7gkx@alap3.anarazel.de
Discussion: https://postgr.es/m/f67b5008-9f01-057f-2bff-558cb53af851@2ndquadrant.com