Use -Wno-format-truncation and -Wno-stringop-truncation, if available.

Use -Wno-format-truncation and -Wno-stringop-truncation, if available.

This back-patches commit e71658523 into out-of-support branches,
pursuant to newly-established project policy.  The point is to
suppress scary-looking warnings so that people building these
branches needn't expend brain cells verifying that it's safe
to ignore the warnings.

Also back-patch the adjacent test from commit 73b416b2e (Suppress
clang's unhelpful gripes about -pthread switch being unused) and
the necessary autoconf macro from fdea2530b.

Discussion: https://postgr.es/m/d0316012-ece7-7b7e-2d36-9c38cb77cb3b@enterprisedb.com