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_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/527e62a5e5d7037780e154159c76cb3f38716c87

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