Call CGUIColor base constructor without C++17 using declaration following rP22558.
This removes the trigger for an unidentified VS2015 bug revealed by the IGUIObject::AddSetting / CGUISetting constructor in rP22604
(unset GUIObject color settings didn't hold the color assigned in the inherited default constructor, so hovered button text appeared black instead of white on Windows).
The code in the two commits seems correct, but VS2015 doesn't seem to implement it correctly (and claims to support C++17) while clang and gcc behave as intended.
Reported By: gameboy, Angen