Finding the pqlib version

I want to add the PQCancel() functionality to PyGreSQL.
Until now, PyGreSQL works with PostgreSQL 7.3+, but
unfortunately, PQCancel() is only available from 8.0+.

In order to not break compilation of PyGreSQL on systems with PG7,
I would like to do something like the following:

#if PQLIB_VERSION_MAJOR < 8  PQRequestCancel(conn)
#else  c = PQgetCancel(conn)  PQCancel(c, errbuf, 256)  PQfreeCancel(c)
#end

However, there is no such thing as "PQLIB_VERSION_MAJOR".
Anything else I could query instead?

-- Christoph