Add support for parsing *FLAGS with shlex

[xbjfk]

Hello,
This PR adds support for parsing *FLAGS with the shlex crate. This effectively fixes #847, and means *FLAGS with spaces designed for meson, make and cmake will work with no extra configuration.
In order to preserve compatibility with existing setups, the behavior must be opted into with CC_PARSE_FLAGS_WITH_SHLEX, or the parse_flags_with_shlex method on the Builder.

[NobodyXu]

Thank you!

just some minor styling suggestions

[NobodyXu]

Thanks LGTM!

Just want @thomcc to double check this, since it involves env var, which is kind of like a new interface, and new API.

Given that we never plan on release v2 I'd like to be a little bit more careful on that.

[thomcc]

I'm wary of adding a dependency since it literally always causes us big problems but I don't see why this one would so I suppose we can give it a shot.

I think I'd prefer this be optional. We probably also shouldn't mention shlex in the interfaces because we could change the implementation to another crate (or no crate).

[xbjfk]

We probably also shouldn't mention shlex in the interfaces because we could change the implementation to another crate (or no crate).

Good idea, I changed the env var to be CC_SHELL_ESCAPED_FLAGS.

I think I'd prefer this be optional

Do you mean the dependency being optional / gated behind a feature? If it might hurt the user experience a bit, but I think it would be fine since callers of cargo who set CFLAGS could use --config

[NobodyXu]

Do you mean the dependency being optional / gated behind a feature?

Given how small shlex is (no dependency, no build-script), I don't think we need to add a feature for it.

In the future we could vendor it, given that the code for splitting based on shell isn't very large, so I don't think it's necessary.

[NobodyXu]

Given that there's no objection, I'd merge it in