Missing libclang.so

[zombiezen]

(Originally reported by @remram44 at tomaka/rustc-emscripten#3)

Building emscripten-sys fails:

error: failed to run custom build command for `emscripten-sys v0.3.2`
process didn't exit successfully: `/src/target/release/build/emscripten-sys-55f5ae5b24c31eac/build-script-build` (exit code: 101)
--- stderr
thread 'main' panicked at 'Unable to find libclang: "couldn\'t find any of [\'libclang.so\', \'libclang.so.1\'], set the LIBCLANG_PATH environment variable to a path where one of these files can be found (skipped: [])"', /checkout/src/libcore/result.rs:916:5

[remram44]

This is not an emscripten-sys issue. I am using it fine since the Docker image was fixed to include that lib, cf https://github.com/asRIA/emscripten-docker/pull/25