Make it possible to use it as rust dependency for a crate that builds a bundled version of a c-lib depending on openssl

[weiznich]

This is rather a long shot feature request. I maintain pq-src, which is a crate bundling the libpq C source code and building + linking it as part of your cargo build process. libpq depends on openssl for TLS encrypted connections. I currently solve that by letting pq-src depend on openssl-src (which statically links openssl) and extracting the relevant information about the include directory from the environment variables they set like this. I would like to be able to do the same thing with rustls-openssl-compant to eliminate the usage of openssl in my dependency tree.

As far as I see that would require:

• Having rustls-openssl-compat be published on crates.io, so that other crates can depend on it. This would then build + link the crate into the final binary, which should export the "right" symbols as far as I can tell
• Make sure that rustls-openssl-compat provides a header file + exposes the location of that header file via an environment variable to other build scripts (See the cargo documentation on this feature here (search for DEP_)
• Likely add support for functions that are used by libpq and that are currently missing from rustls-openssl-compat, but that's secondary, as I would like to get your opinion on this idea first