pgsql: Fix configure's AC_CHECK_DECLS tests to work correctly with clan

Fix configure's AC_CHECK_DECLS tests to work correctly with clang.

The test case that Autoconf uses to discover whether a function has
been declared doesn't work reliably with clang, because clang reports
a warning not an error if the name is a known built-in function.
On some platforms, this results in a lot of compile-time warnings about
strlcpy and related functions not having been declared.

There is a fix for this (by Noah Misch) in the upstream Autoconf sources,
but since they've not made a release in years and show no indication of
doing so anytime soon, let's just absorb their fix directly. We can
revert this when and if we update to a newer Autoconf release.

Back-patch to all supported branches.

Discussion: https://postgr.es/m/26819.1542515567@sss.pgh.pa.us

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/8af1511e9cb5fbcfe47cb081ff9a1800fee73348

Modified Files
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aclocal.m4 | 1 +
config/check_decls.m4 | 116 ++++++++++++++++++++++++++++++++++++++++++++++++++
configure | 100 +++++++++++++++++++++++++++++++++++++++++--
configure.in | 5 ++-
4 files changed, 217 insertions(+), 5 deletions(-)