Rework build.rs and support linking to system RocksDB

[lovesegfault]

This PR completely reworks librocksdb-sys's build system. Here are the main achievements:

• Complete separation of vendoring and system linking logic
• Feature-based selection of linking mode (static or dynamic)
• Feature-based selection of vendoring (defaults to on)
• Helpful error messages with panics
• Simplifies code where reasonable

With this I hope our vendoring and linking stories become clearer.

cc. @jonhoo

Fixes #310

[jonhoo]

This is awesome stuff @lovesegfault!

[vitvakatu]

Thank you, looks good in general. Could you document vendor feature somewhere? Ideally, in the crate docs, so it would be visible on docs.rs

[grtlr]

Thanks for the awesome work @lovesegfault! Is there any status update on this PR, or is there another way to use the system's RocksDB? RocksDB is one of the biggest chunks of our compile time—we'd be happy to shave some part of it off.

[Ralith]

Would love to see this picked back up.

[Iamknownasfesal]

Any updates about this? We would love this change

[DaveWK]

reposting here for anyone else who happens to find this pr via search -- you can use the precompiled rocksdb from your package manager by setting ROCKSDB_LIB_DIR (in my case for fedora 38, ROCKSDB_LIB_DIR=/usr/lib64

[Xuanwo]

Hi, @aleksuss, would you like to take another review of this PR? I'm willing to continue the work to get it merged.

[aleksuss]

@Xuanwo ok. I will look at this when the merge conflicts will be resolved.

[aleksuss]

@Xuanwo, I like the idea overall, but I guess the PR should be updated.

[Xuanwo]

@Xuanwo, I like the idea overall, but I guess the PR should be updated.

I will continue the work.

[jhult]

@Xuanwo, any news?

[Xuanwo]

@Xuanwo, any news?

Apologies for not having enough time for this. Feel free to take over if you're interested.

[spdfnet]

Don't want t cause any flame wars, but the future might be https://github.com/speedb-io/rust-speedb