wrong configuration key in `build.rs` and `Cargo.toml`

[luozhonghe]

The wrong configuration key (libavs) seems to be defined in both build.rs and Cargo.toml files. The FFmpeg/configure file in the official FFmpeg/v0.8 repository only appears to define the libxavs key. So there is no shared library named libavs.

rust-ffmpeg-sys/Cargo.toml

Line 96 in 36eca19

build-lib-avs = ["build"]

rust-ffmpeg-sys/build.rs

Line 530 in 36eca19

enable!(configure, "BUILD_LIB_AVS", "libavs");

https://github.com/zmwangx/rust-ffmpeg/blob/118f7e7132aee9568340ec817672bd1ed2a095aa/Cargo.toml#L88

https://github.com/FFmpeg/FFmpeg/blob/a064b4eb1297a9bfaff63a35a398d417008a88bf/configure#L2958