Allow defining SOCI_POSTGRESQL_NOSINLGEROWMODE for PostgreSQL < 9
Since the changes of b70a4e8b, PostgreSQL backend is not compatible with PostgreSQL < 9, contrary to the documented supported platforms in the documentation. Ideal would be to continue supporting the old versions by default and use PQsetSingleRowMode() only if it's detected as being available by CMake, but for now at least allow disabling single mode support manually. Closes #571.
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