Don't require libclang by default

[matklad]

It seems like libappindicator-sys requires libclang to build (possibly due to bindgen):

14:34:30|~/projects/systray-rs|master⚡?
λ cargo run --example systray-example
   Compiling futures-executor v0.3.5
   Compiling libappindicator-sys v0.4.0
error: failed to run custom build command for `libappindicator-sys v0.4.0`

Caused by:
  process didn't exit successfully: `/home/matklad/projects/systray-rs/target/debug/build/libappindicator-sys-bbedb3ed9e28adf0/build-script-build` (exit code: 101)
--- stdout
cargo:rustc-link-lib=appindicator3
cargo:rustc-link-search=native=/nix/store/qm1h65bw28h0gffcagq88csh0hl8ka15-gtk+3-3.24.13/lib
cargo:rustc-link-search=native=/nix/store/dara95ay6r49srnia5fllxg84n4fr3qc-atk-2.34.1/lib
cargo:rustc-link-search=native=/nix/store/3h14bkgm9har3rdds7924drn75plnhla-glib-2.62.4/lib
cargo:rustc-link-search=native=/nix/store/5nbgbfc68hfmy6hqr9iap8jpl0mb6882-cairo-1.16.0/lib
cargo:rustc-link-search=native=/nix/store/6x2ypk1p1kbij71cdwd92vxpi1jx923z-gdk-pixbuf-2.40.0/lib
cargo:rustc-link-search=native=/nix/store/aqy2zrf4ax7nab6vbfacns9708bhvly2-pango-1.44.7/lib
cargo:rustc-link-search=native=/nix/store/m0j3kjgrb9v9zbkr0m48layw1lj4ssbl-harfbuzz-2.6.4/lib
cargo:rustc-link-search=native=/nix/store/c5w9axdm5damqb0qrjn7rn54q4h3gvjd-libappindicator-gtk3-12.10.0/lib
cargo:rustc-link-search=native=/nix/store/qd126mlckjg6wk1v8zmbvnr3nrlwb26g-libdbusmenu-gtk3-16.04.0/lib
cargo:rustc-link-lib=appindicator3
cargo:rustc-link-lib=dbusmenu-glib
cargo:rustc-link-lib=gtk-3
cargo:rustc-link-lib=gdk-3
cargo:rustc-link-lib=pangocairo-1.0
cargo:rustc-link-lib=pango-1.0
cargo:rustc-link-lib=harfbuzz
cargo:rustc-link-lib=atk-1.0
cargo:rustc-link-lib=cairo-gobject
cargo:rustc-link-lib=cairo
cargo:rustc-link-lib=gdk_pixbuf-2.0
cargo:rustc-link-lib=gio-2.0
cargo:rustc-link-lib=gobject-2.0
cargo:rustc-link-lib=glib-2.0

--- stderr
thread 'main' panicked at 'Unable to find libclang: "couldn\'t find any valid shared libraries matching: [\'libclang.so\', \'libclang-*.so\', \'libclang.so.*\'], set the `LIBCLANG_PATH` environment variable to a path where one of these files can be found (invalid: [])"', /home/matklad/.cargo/registry/src/github.com-1ecc6299db9ec823/bindgen-0.52.0/src/lib.rs:1895:13
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

warning: build failed, waiting for other jobs to finish...
error: build failed

It seems like this shouldn't be the case though? Would it be possible to vendor generated code ? I would think that libappindicator is ABI-stable.