Specify C++17 requirement explicitly

[eXpl0it3r]

While I guess it's implicitly set by using SFML, it makes sense to have this option explicitly set on the target executable.

[ChrisThrasher]

Given that the executable already inherits this requirements from SFML’s CMake targets, what do we gain by repeating it explicitly?

[eXpl0it3r]

That it is explicit.

If someone switch to find_package stuff suddenly breaks, even though this should largely be interchangeable.
Personally, I'd never write my own application that I know uses C++17 and not specify that in my own CMake file, just because it's already implicitly imported from some library.

[ChrisThrasher]

Switching to find_package won’t break this either. No matter how you link to our CMake targets you’ll inherit this requirement. You’d have to modify this script very heavily for this to become a problem. That being said this is an innocuous change and it makes it marginally easier to upgrade to C++20 so I’m fine with it.