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@technowledgy.de>
Discussion: https://www.postgresql.org/message-id/flat/427c7c25-e8e1-4fc5-a1fb-01ceff185e5b@technowledgy.de