pgsql: Add warning option -Wold-style-declaration

Add warning option -Wold-style-declaration

This warning has been triggered a few times via the buildfarm (see
commits 8212625e53f, 2b7259f8557, afe86a9e73b), so we might as well
add it so that everyone sees it.

(This is completely separate from the recently added
-Wold-style-definition.)

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/aa73q1aT0A3/vke/%40ip-10-97-1-34.eu-west-3.compute.internal

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c73e8ee061adf26ed77ffa7e676f5de6d9f6da21

Modified Files
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configure    | 41 +++++++++++++++++++++++++++++++++++++++++
configure.ac |  2 ++
meson.build  | 14 +++++++++-----
3 files changed, 52 insertions(+), 5 deletions(-)