Default emit_rerun_if_env_changed to true

[thomcc]

As mentioned in #701 (comment), I don't think there's a reason to default this to false. rerun-if-env-changed is purely additive (unlike rerun-if-changed, which has a fairly good default cleared if you emit it at all), so it should never hurt.

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I've also added code to skip emitting it for vars that provided by cargo, otherwise every use of cc will end up with cargo:rerun-if-env-changed invocations for pointless things like DEBUG, OPT_LEVEL, and so on. I don't think these are likely to hurt anything, but they definitely don't help, and cargo's docs say not to.

An alternative to this would be to add a different function for vars provided by cargo, but that sounds like a headache to maintain, as reviewers could easily not notice the wrong function getting called for a given variable.

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I'd like to get this in before the release, but mostly for the change of the default (since changing this after the release would be breaking) rather than skipping the cargo vars (which is not actually necessary).

[JohnTitor]

Thanks for the explanation, makes sense to me 👍

[briansmith]

I think this is a breaking change. In my CI, I was (perhaps wrongly) setting a bunch of environment variables when doing cargo test but then not setting them when I ran cargo test --doc immediately after. Previously this worked fine because it didn't re-run my build script (and so didn't re-run anything using cc-rs) to run the doctests but afterwords my CI broke. I have fixed it on my end but I want to notify you and others that this is breaking for some as I think I that might be a surprise.