minor cleanups to ffmpeg build setup

* the "lib: {}" print didn't do anything. It turns out that the pkg-config
  crate emits the necessary metadata for linking automatically. I had the
  wrong format and didn't notice because something else did it correctly.

* gcc::Config is deprecated; the new name is Build.

* and the crate is now called cc, version 1.0.

Stuff found while looking at #11. Still haven't figured that issue out.

ffmpeg/Cargo.toml

@@ -13,5 +13,5 @@ libc = "0.2"
 log = { version = "0.3", features = ["release_max_level_info"] }
 
 [build-dependencies]
-gcc = "0.3"
+cc = "1.0"
 pkg-config = "0.3"