Drop PDO support for extremely old libpq versions

configure will now fail if any of the following function is missing:
* PQprepare
* PQexecParams
* PQescapeStringConn
* PQescapeByteaConn
Deprecated PDO::PGSQL_ATTR_DISABLE_NATIVE_PREPARED_STATEMENT
Added new PDO::PGSQL_ATTR_DISABLE_PREPARES that uses PQexecParams

Faster than prepared statements when queries are run once. Slightly
slower than PDO::ATTR_EMULATE_PREPARES but without the potential
security implications of embedding parameters in the query itself.
Updated NEWS
Merge branch 'PHP-5.6'

* PHP-5.6:
  Added new PDO::PGSQL_ATTR_DISABLE_PREPARES that uses PQexecParams
  Deprecated PDO::PGSQL_ATTR_DISABLE_NATIVE_PREPARED_STATEMENT
  Drop PDO support for extremely old libpq versions