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_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/03e749c2b89e880e0a226458e9446fca4440696f

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