Previously, the update complained due to missing migration files:
db=# alter extension plv8 update to '2.3.15';
ERROR: extension "plv8" has no update path from version "2.3.13" to version "2.3.15"
Now it fails for a different reason:
db=# alter extension plv8 update to '2.3.15';
ERROR: could not access file "$libdir/plv8-2.3.13": No such file or directory
Changing installation directory using DESTDIR is an ugly hack that might backfire
but PGXS does not currently seem to allow overriding the installation directory.
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