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_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/3b0f20f6ce145b0cd5aefa147a7e425b60601a58

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