PGDG debs not playing nice with pgxn (bug?)

I attempted to install pg_repack on a server running PostgreSQL 9.1.13
on Ubuntu 12.04. As I didn't find pg_repack in the repository or
installed extensions, I decided to use pgxn.

I installed pgxnclient from PGDG. To use pgxn I had to install
postgresql-server-dev-9.1 which installed, as a requirement, libpq-dev.
The pgxn install failed with "error: unknown type name 'pg_int64'" as
described in this thread:
http://www.postgresql.org/message-id/53989C91.6050403@pinpointresearch.com

Further investigation revealed that the only seemingly available
libpg-dev in the repo is version 9.3.4. The end result is that there are
multiple versions of postgres_ext.h automatically installed and that
pgxnclient as installed from the PGDG repo chooses the wrong one:
/usr/include/postgresql/postgres_ext.h
/usr/include/postgresql/9.1/server/postgres_ext.h

Cheers,
Steve