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/[email protected]

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e716585235b175de767d3b210f0a963457dfa77f

Modified Files
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configure | 88 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
configure.in | 12 +++++++++
2 files changed, 100 insertions(+)