Language standards not set in vsxmake project settings

[paul-reilly]

I noticed Intellisense errors in a vsxmake project I opened.

It turns out that the language standard set with set_languages() is not being set in the generated VS project.

Specifically, I set set_languages("c++17") and the VS 2019 project still has C++14.

[waruqi]

Ok, I will look at it.

[waruqi]

I have supported it on dev branch, you can try it again.