Thread: 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.

Back-port commit 16fbac39f into 9.3 and 9.2, so that these
out-of-support branches can be built with clang without wading
through a pile of warnings about strlcpy and related functions.
check_decls.m4 required some adaptation to work with autoconf 2.63,
but nothing too major.

Discussion: https://postgr.es/m/26819.1542515567@sss.pgh.pa.us
Discussion: https://postgr.es/m/1081321.1663775084@sss.pgh.pa.us

Branch
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REL9_2_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/b8464d2f5da0d1abe0925c5712d0c4810986e629

Modified Files
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aclocal.m4            |   1 +
config/check_decls.m4 | 112 +++++++++++++++++++++++++++++
configure             | 192 +++++++++++++++++++++++++++++++++++++++++++++++---
configure.in          |   5 +-
4 files changed, 300 insertions(+), 10 deletions(-)