Support for pkgconfig / dynamically linking with system freetype2 (accidentally?) removed from 0.17.0

The support for using pkg-config and dynamically linking with system freetype2 was removed with this PR: https://github.com/PistonDevelopers/freetype-sys/pull/106 The commit message and PR don't mention pkg-config support at all, but it was removed alongside the cmake support. Would it be possible to restore this functionality? Statically linking against a vendored copy is usually forbidden for official Linux distribution packages, so the pkg-config functionality that was present until 0.16.0 was really nice for that :(