pgsql: Make building with LTO work on macOS

Make building with LTO work on macOS

When building with -flto, the backend binary must keep many otherwise
unused symbols to make them available to dynamically loaded modules /
extensions. This has been done via -Wl,--export-dynamic on many
platforms for years. This flag is not supported by the macOS linker,
though. Here it's called -Wl,-export_dynamic instead.

Thus, make configure pick up on this variant of the flag as well.
Meson has the logic upstream as of version 1.5.0.

Without this fix, building with -flto fails with errors similar to [1]
and [2].

[1]: https://postgr.es/m/1581936537572-0.post%40n3.nabble.com
[2]: https://postgr.es/m/21800.1499270547%40sss.pgh.pa.us

Author: Wolfgang Walther <walther(at)technowledgy(dot)de>
Discussion: https://www.postgresql.org/message-id/flat/427c7c25-e8e1-4fc5-a1fb-01ceff185e5b(at)technowledgy(dot)de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/73275f093f8969f4d2353fcc803377350aa650f6

Modified Files
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configure | 43 +++++++++++++++++++++++++++++++++++++++++++
configure.ac | 6 ++++++
2 files changed, 49 insertions(+)