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_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/a13b63c788a30a49676ddee5ce3ab2bcd1c811d1

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