Tom,
I just tested and yes that worked. Once we have the new library for the hostname, pg_upgrade is not complaining about the hostname extension.
Another thing we found is this. We needed to drop json_build extension before the upgrade. However the upgrade fails with the following error and the way to resolve it is to install json_build94 library. Any ideas why this might be?
/usr/pgsql-9.4/bin/pg_upgrade --check --link
...
....
Your installation references loadable libraries that are missing from the
new installation. You can add these libraries to the new installation,
or remove the functions using them from the old installation. A list of
problem libraries is in the file:
loadable_libraries.txt
Failure, exiting
[postgres@pdxdvrptsrd04 ~]$ cat loadable_libraries.txt
Could not load library "json_build"
ERROR: could not access file "json_build": No such file or directory
Thanks
Smitha