CXX_STANDARD 17

[syncingdisks]

Please default to CXX_STANDARD 17 on cegui/cmake/CEGUIMacros.cmake instead of 14.

[manhnt9]

It depends on the platform. For example embedded / mobile systems are not necessarily using C++17.
Even on desktop C++17 isn't very popular as well. Many people's projects are still using C++14 or even strictly C++11.
Personally I think that C++14 is still for me in several years.

[syncingdisks]

Don't get me wrong, `personally' is not one of my wet dreams to have CXX_STANDARD 17 but errors like:

cegui/cegui/include/CEGUI/InputEvent.h:47:130: error: 'is_enum_v' is not a member of 'std'; did you mean 'is_enum'?

cegui/cegui/include/CEGUI/InputEvent.h:47:140: error: template argument 1 is invalid

using the defaults on 14 are a show stopper for newcomers.

On another note, keep up the good work CEGUI team!

[manhnt9]

Well tbh I'm kinda questioning recent modern C++ features added to the codebase too. Haven't had time to really pull the new version yet to review but I think that it should be optimized since there're multiple related issue tickets have been shown.

[niello]

Fixed unintended C++17 usage, now must build under C++14