pgsql: Disable clang 16's -Wcast-function-type-strict.

Disable clang 16's -Wcast-function-type-strict.

This is a back-patch of commit 101c37cd into REL_14_STABLE and
REL_15_STABLE.  Those branches had commit de8feb1f3, which turned on
-Wcast-function-type, but did not disable -Wcast-function-type-strict.
This silences warnings about function pointer types without prototypes
based on new C23 rules, that we want to suppress for now.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CA%2BhUKGKnBvdrbH2LW%2B7-Lv599t9JFOHjx%3Dxw-VQmdoj%3D9585CQ%40mail.gmail.com
Discussion: https://postgr.es/m/CA%2BhUKGJvX%2BL3aMN84ksT-cGy08VHErRNip3nV-WmTx7f6Pqhyw%40mail.gmail.com

Branch
------
REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/6e2552d180a0a3a694c2f91ec01bec51d9fed671

Modified Files
--------------
configure    | 44 ++++++++++++++++++++++++++++++++++++++++++++
configure.ac |  6 ++++++
2 files changed, 50 insertions(+)