Fix wrong qreal architecture checks

Since Qt 5.2, qreal is double on all architectures by default,
but some vendors (like Jolla) still use qreal = float.

Instead of old (and broken) architecture checks, use runtime
check sizeof(qreal) == sizeof(double).

Remove the compile-time checks between DBL_MIN/DBL_MAX checks,
these are sufficient on their own.

Change-Id: Iae0c6486afaed0f0653fb3338c048c8f22d1e587
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>
Reviewed-by: Lisandro Damián Nicanor Pérez Meyer <perezmeyer@gmail.com>