PQescapeIdentifier
appeared in PostgreSQL 9.0, which corresponds to libpq.so.5.3
9.0 release notes have this entry:
Add libpq functions PQescapeLiteral() and PQescapeIdentifier() (Robert Haas)
You seem to use client libraries from PostgreSQL-8.4 (libpq.so.5.2
), that would be why it lacks this function.