Add method for specifying C/C++ standard version

[hikari-no-yume]

As requested by #565.

Please have a look at the documentation I have included. :)

I am not doing any abstraction over special cases like the name for the work-in-progress standard version. It's just the basics.

I want to propose adding this to cc because:

• It should be useful fairly often, considering that using the right standard version can be critical to compiling modern code. I have a C++17 dependency in one of my projects, and I couldn't build my code interfacing with it without specifying the standard version, because even very new compilers seem to default to older versions. As far as I know, the compatibility between C++ versions isn't perfect, so that might be why.
• It can save people the effort of having to consult both GCC/Clang and MSVC documentation, and improve the portability of code from people who haven't tested on Windows:
  • The GCC/Clang and MSVC flag syntaxes are not quite the same, so without support in the cc crate, the developer has to write conditional, platform-specific code.
  • The common subset of supported versions between GCC/Clang and MSVC is not something you can intuitively guess. The documentation added here can help.
• While I haven't done that here, it could be enhanced in future to handle special cases like the different names for the WIP future C and C++ standards.

[thomcc]

This looks fine to me.

[thomcc]

There are conflicts I guess. Can you rebase?

[hikari-no-yume]

Sure, I can do that later today.

Maybe I can take that opportunity to edit the doc comment a bit to address the review comment above.

[thomcc]

Sure. I think it's fine as-is but additional clarification wouldn't hurt.

[hikari-no-yume]

I rebased it now. The conflict was caused by the code being changed to use Arc<str> instead of String for all the other fields. I'm not sure whether that has any benefit for this field, but I changed it to match. I also clarified the thing with MSVC and C++11.

[thomcc]

Thanks.

[hikari-no-yume]

Thanks for taking the time to review and merge this! :)

[hikari-no-yume]

@thomcc Oh, maybe you'll want to close #565 now.