Re: Meson add host_system to PG_VERSION_STR

Given we're looking at improving this, should we also add 32/64-bit piece?

If so, we probably should move building PG_VERSION_STR to later so we can use
SIZEOF_VOID_P - configure.ac does that too.

With extra_version set to -andres the attached results in:

PostgreSQL 16devel-andres on x86_64-linux, compiled by gcc-13.0.0, 64-bit