Put bindgen behind a seperate feature

[weiznich]

This commit changes proj-sys to not depend on bindgen by default as that introduces a quite heavy build time dependency (libclang) which might not be there on all systems. Instead bundled bindings for the proj version build by the bundled_proj feature are provided.

[urschrei]

I don't know whether there's a prevailing best practice when it comes to sys crates, but my assumption has always been that the crate defaults to trying to use system libraries and the bundled library is an option / fallback. There's also a technical question around the bundled bindings: do they work correctly cross-platform? on 32-bit vs 64-bit?

[weiznich]

At least most of the *-sys crates I've interacted with provide bundled bindings by default as that remove the dependency on libclang.

Examples:

• libsqlite3-sys
• pq-sys
• mysqclient-sys
• openssl-sys
• netcdf-sys
• libz_sys
• gdal_sys

Some of them provide the possibility to use bindgen at build time to build your own bindings instead.

There's also a technical question around the bundled bindings: do they work correctly cross-platform? on 32-bit vs 64-bit?

That depends on the header as far as I can tell. If it's written in a cross-platform compatible way it does work otherwise there might be issues for 32-bit vs 64bit or for windows vs linux or …

Some of the crates above handle that by just providing different bindings for different platforms and conditionally including the right bindings for the target. Other provide the some feature flag to run bindgen at buildtime for that, and others do both.

[weiznich]

Is there any interest in moving this PR forward from the relevant maintainers?

[weiznich]

@urschrei @michaelkirk @lnicola What is required to move this forward?

[lnicola]

In gdal-sys we bundle pre-built bindings for every minor GDAL version and I don't think we've ran into any compatibility issues (except on Windows where I'm not convinced that people manage to use it).

I don't know the ABI compatibility story of GDAL, but I doubt that they are backwards-compatible.

[weiznich]

@urschrei @michaelkirk @lnicola This is another gentle ping if there is any interest in moving this forward