Adjust cbindgen Overrides for CFFI

[nhusung]

My Use-case

I'm working on a project for which I'd like to create C/C++ and Python bindings. My gut feeling says that having a simple hand-written C FFI with a C header generated by cbindgen is the best option. On top of that, I'd like to provide user-friendly, idiomatic (hand-written) C++ and Python interfaces. Maturin seems to be a good fit for simplifying the build process of the Python bindings.

Issue

For a good integration in C/C++ projects, I use cbindgen options such as sys_includes (generating #include <stdlib.h>, …) cpp_compat (#ifdef __cplusplus etc.). Currently, Maturin uses the same cbindgen.toml modulo the overrides for defines and include_guard, and passes the resulting header to CFFI. Unfortunately, CFFI's cdef method cannot deal with those C preprocessor directives generated under my configuration, resulting in a crash.

Proposed Change, Alternatives

This PR adds a few more overrides: sys_includes = [] and cpp_compat = false are the relevant ones for my use-case. I also disabled pragma_once (I'm not sure if the #pragma once can cause issues, though). language = C and no_includes = true were specified in the eprintln! message above but not actually set, so I added them as well. (In my tests, no_includes = true does not make sys_includes = [] obsolete.)

The proposed change does the job for me, but I'm not sure if others might need additional overrides. In the cbindgen.toml documentation at https://github.com/mozilla/cbindgen/blob/master/docs.md#cbindgentoml, there are various options that might cause problems, e.g., function attributes such as NO_RETURN, which is emitted for Rust functions returning !. I could imagine two alternatives/additions to this PR that might help in such situations: (1) Providing an option to manually set the cbindgen.toml path or (2) exposing the override settings. I'm not sure if this is needed, though; this PR makes things work out of the box in my scenario.

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[messense]

I'm not totally sure about this, please review this if you have time @konstin, thanks!