Add version features
Add version features

- Installed library version detection no longer impacts conditional
  compilation.
- Each crate defaults to supporting a particular minimal upstream
  version.
- Higher (newer) versions need to be enabled via cargo features e.g.
  `--features 3.16`, `features = ["3.16"]`.
- The selected version is checked against `pkg-config` aborting the
  build if it's too high.
- The use of `pkg-config` can be overridden by setting `GTK_LIB_DIR` env
  variable, which translates into `-L $GTK_LIB_DIR` linker argument. No
version checks are done in this case.

gkoz/gtk-rs-pango