queriing the version of libpq

With 9.0 there is the new hex encoding for BYTEA, which is activated by
default.

libpq BEFORE 9.0 are not able to decode that encoding.

I am programming with Python, using psycopg2. When psycopg2 is linked with
libpq 9.0, everything works fine with hex-encoding; if psycopg2 is linked
with libpq < 9.0, decoding hex-encoded bytea fails (it stays hexencoded).

This happens because in default configuration psycopg2 calls the
libpq-decode-encoded-bytea function (which is the way it should be done).

Now I would love to have an additional check "is the used psycopg2 linked to
an advanced-enough libpq", to be able to set bytea_output to 'escape' if the
libpq is not worthy.

My question: Which way is available to query the linked libpq version?

My other option is to select 'something_that_gets_casted_to_bytea'::bytea,
and check the return value. BUT that requires a round-trip to the server....

Harald
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