Update lmdb to latest version and other goodies

[victorporof]

This PR:

• Updates lmdb to 0.9.24 (with MOZ changes on top)
• Makes the sys crate create bindings automatically using rust-bindgen
• Allows cc to handle build optimizations, fixes build mdb.c without optimizations for debug build #54
• Hides unused parameter warnings while building lmdb

[mykmelez]

This looks great; I'm super-psyched that you've automated the bindings generation.

I looked at a couple of the failing Windows builds, and they fail with errors like:

cargo:warning=cl : Command line error D8021 : invalid numeric argument '/Wno-unused-parameter'

Setting the flag via https://docs.rs/cc/1.0.4/cc/struct.Build.html#method.flag_if_supported should resolve that.

I'm unsure about the errors on macOS, but it looks like they only occur with Rust 1.30, so perhaps a dependency requires a newer version. Maybe we can simply drop support for that version of Rust?

[mykmelez]

Nice!

[victorporof]

I think we need to install libclang on windows:

2019-08-31T06:28:25.0361872Z --- stderr
2019-08-31T06:28:25.0362101Z thread 'main' panicked at 'Unable to find libclang: "couldn\'t find any valid shared libraries matching: [\'clang.dll\', \'libclang.dll\'], set the `LIBCLANG_PATH` environment variable to a path where one of these files can be found (invalid: [])"', src\libcore\result.rs:999:5
2019-08-31T06:28:25.0362841Z note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.

[mykmelez]

I think we need to install libclang on windows:

Ah, indeed, according to https://rust-lang.github.io/rust-bindgen/requirements.html. And the LLVM installer for Windows is GUI-based, which'd make it hard to install in CI. Perhaps instead of regenerating bindings at build time we should include the generated bindings in this changset?