Detect missing qt ssl feature and fail early

[Abestanis]

This might have helped in #627.

The sslConfiguration member is gated behind a #if QT_CONFIG(ssl) check:

#if QT_CONFIG(ssl)
QSslConfiguration sslConfiguration() const;
void setSslConfiguration(const QSslConfiguration &configuration);
void ignoreSslErrors(const QList<QSslError> &errors);
#endif

According to the official Qt6 build system documentation, we can check for the ssl feature by checking the QT_FEATURE_ssl variable.