meson: Fix installation path computation
We have the long-standing logic to append "postgresql" to some
installation paths if it does not already contain "pgsql" or
"postgres". The existing meson implementation of that only considered
the subdirectory under the prefix, not the prefix itself. Fix that,
so that it now works the same way as the implementation in
Makefile.global.
Reviewed-by: Andres Freund <[email protected]>
Discussion: https://www.postgresql.org/message-id/
a6a6de12-f705-2b33-2fd9-
9743277deb08@enterprisedb.com