Reexport Faiss version in c_api

[ava57r]

Summary

Hello.

Library supports version defines here
https://github.com/facebookresearch/faiss/blob/master/faiss/Index.h#L19

I think this functionality will be useful in c_api.

[Enet4]

For additional context, this has been brought up at faiss-rs here, as a potential aid at determining whether some feature can be used by the given bindings.

I tried to understand how Python exports the version of Faiss. I suspect that all it does is hard-code the version from the header file via SWIG.
We could also copy those symbol declarations into one of the headers of the C API through some means, but maybe there is interest in embedding the version into the implementation objects instead (.a / .so). This would allow bindings to provide support for a range of versions, although currently this poses some risks due to versions not being semver compliant.

For the time being, it would be an improvement for users of the C API to have access to the same symbol definitions.

[mdouze]

Right, we should think about this.

[ava57r]

Other version
https://github.com/facebookresearch/faiss/blob/master/CMakeLists.txt#L10
than
https://github.com/facebookresearch/faiss/blob/master/faiss/Index.h#L19

[mdouze]

Right, let's make an API entry to return the version (in the same fashion as get_compile_options() )

[junjieqi]

#3688

now the version is supported from C_API