Explicit method to specify C/C++ standard

[alexeyr]

It would be nice to specify the standard explicitly as a method on Build, e.g. Build::new().standard("c++14"). This abstracts from difference between GCC/Clang -std= and MSVC /std:. Questions to answer:

1. What to do if you pass gnu14 on MSVC: translate to the closest c++14 or give an error?
2. Similarly, MSVC doesn't yet support /std:c++20 and requires /std:c++latest for C++20 features. Should standard("c++20") test if /std:c++20 works and use /std:c++latest if it isn't?

[hikari-no-yume]

The simplest thing you can do right now is something like this:

if build.get_compiler().is_like_msvc() {
    build.flag("/std:c++17")
} else {
    build.flag("-std=c++17")
}

But it doesn't solve the issues you mention with unusual versions…

Do you think it's be useful to have this even if it only supports the simple cases? If so, I might contribute a fix for this myself, because I need this flag for one of my projects.

[hikari-no-yume]

I decided to just do it :) #761

[thomcc]

Fixed by #761, will be in the next release.