pgsql: Use -Wno-format-truncation and -Wno-stringop-truncation, if avai

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

gcc 8 has started emitting some warnings that are largely useless for
our purposes, particularly since they complain about code following
the project-standard coding convention that path names are assumed
to be shorter than MAXPGPATH. Even if we make the effort to remove
that assumption in some future release, the changes wouldn't get
back-patched. Hence, just suppress these warnings, on compilers that
have these switches.

Backpatch to all supported branches.

Discussion: https://postgr.es/m/1524563856.26306.9.camel@gunduz.org

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/119290be6fe3bea12053042f5d3eaa135a260a52

Modified Files
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configure | 80 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
configure.in | 12 +++++++++
2 files changed, 92 insertions(+)