pgsql: Log LLVM library version in configure output.

Log LLVM library version in configure output.

When scanning build farm results, it's useful to be able to see which
version is in use.  For the Meson build system, this information was
already displayed.

Back-patch to all supported branches.

Discussion: https://postgr.es/m/4022690.1697852728%40sss.pgh.pa.us

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/0e49e23782c55237f54aa60ed7a2b755b47cd4a2

Modified Files
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config/llvm.m4 | 1 +
configure      | 2 ++
2 files changed, 3 insertions(+)