Use proper feature macro test to disable constexpr
Use proper feature macro test to disable constexpr

With C++11 the constexpr usage is ill-formed...

Refs: gh-212

Signed-off-by: Matthias Kretz <372ea4eca737a13cd010ee806d4ce7b98ee71550@kde.org>

VcDevel/Vc,VcDevel/Vc,VcDevel/Vc,VcDevel/Vc