Use freetype-sys on Linux.

See:
https://github.com/servo/libfreetype2/pull/2
https://github.com/servo/skia/pull/38
https://github.com/servo/rust-azure/pull/114

Let’s close #3867, even though this only works around and doesn’t fix the underlying issue.
List bzip2 as a requirement in README.md

… but not for Homebrew or MacPorts, OS X which apparently ships with it.
auto merge of #3980 : SimonSapin/servo/freetype-sys-linux, r=larsbergstrom

See:
https://github.com/servo/libfreetype2/pull/2
https://github.com/servo/skia/pull/38
https://github.com/servo/rust-azure/pull/114

Let’s close #3867, even though this only works around and doesn’t fix the underlying issue.

r? @larsbergstrom