Create constructor allowing user to set c_cfg and cxx_cfg

[lthiery]

In a previous PR, an extra Config field and a setter for that field were created.

Perhaps letting the user pass c_cfg and/or cxx_cfg would allow more flexible usage? Otherwise, every "useful" configuration field could potentially be made into a field and a setter.

[alexcrichton]

Thanks for the PR! Could you expand a bit on the motivation for this? I'd prefer to avoid making cc part of the public API here if possible.

[lthiery]

I don't feel strongly about it, but it was just an idea I had following the previous PR. Rather than adding a new Config field and a new setter for every "useful" c[xx]_cfg option, there could be a public API to initialize the c[xx]_cfg with a preconfigured c[xx]_cfg.

This just adds a huge degree of flexibility, but maybe that's not as important as obscuring cc from the public API.

[alexcrichton]

Ah ok, in that case I'd prefer to keep this as-is and reexport options here as necessary, but thanks for the PR!