Generate a Makefile.global that's complete enough for PGXS to work for some
extensions. It is likely that this compatibility layer will not suffice for
every extension and not all platforms - we can expand it over time.
This allows extensions to use a single buildsystem across all the supported
postgres versions. Once all supported PG versions support meson, we can remove
the compatibility layer.
Reviewed-by: Peter Eisentraut <peter.eisentraut@enterprisedb.com>
Discussion: https://postgr.es/m/20221005200710.luvw5evhwf6clig6@awork3.anarazel.de
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