Link with system rocksdb and snappy libs through envvars

[ozkriff]

This PR adds 4 env variables: ROCKSDB_LIB_DIR, SNAPPY_LIB_DIR, ROCKSDB_STATIC and SNAPPY_STATIC.

What do you think of this? :)

(related to #138)

[stanislav-tkach]

I want to add more details: we are currently using fork of rust-rocksdb for our project, but we prefer to switch to your crate. Still we need several things to be done and this is one of them. Thus, we are very interested in any feedback.

[stanislav-tkach]

@spacejam Sorry for bothering, but can you please give (any) feedback on this?

[spacejam]

Sorry for the huge delay on this! Just started a new job and I've been getting used to a new schedule. I like this approach! I'll cut a new version in a few minutes that includes this change :)

[spacejam]

I've published rocksdb 0.10.0 which includes these changes!

[stanislav-tkach]

@spacejam Thank you! Probably, there will be several more things that we need before switching to your crate, but we are looking forward to that.

[jonhoo]

It'd be really neat if rocksdb-sys checked if rockdb and snappy were available in the standard OS locations too. It's a little unfortunate to have to pass custom environment variables every time we build even though those libraries are installed system-wide.

EDIT: Or even just a ROCKSDB_USE_SYSTEM=1 to link without checking?

[jonhoo]

Tangentially related to my comment above — this should probably be mentioned in the docs.