Fixing security vulnerability with Qt 4.8.5+ and PostgreSQL.

Properly detects whether Qt performs slash escaping in SQL queries or
not, and then configures PostgreSQL accordingly. This bug was a
introduced due to a bugfix in Qt 4.8.5 disables slash escaping when
binding queries: https://bugreports.qt-project.org/browse/QTBUG-30076
Thanks to brot and Tucos.

[Fixes #1244]