title: How can I install and connect the alsa.pc when building the 'alsa-sys' crate as a dependency?
body: <p>I'm trying to build a rust binary using travis-ci. I managed to get the windows and osx build working, but the linux build keeps failing.</p>
  
  <p>It seems the 'alsa-sys' crate, one of the dependencies, cannot locate the alsa.pc in the linux-vm.</p>
  
  <p>.travis.yml:</p>
  
  <pre><code>language: rust
  cache: cargo
  rust:
  - stable
  
  matrix:
    allow_failures:
    - rust: nightly
    fast_finish: true
  os:
  - osx
  - windows
  - linux
  
  script:
    - cargo build --verbose --release
  
  before_deploy:
  - if [[ -f target/release/soundsense-rs ]]; then mv target/release/soundsense-rs "target/release/soundsense-rs-$TRAVIS_TAG-$TRAVIS_OS_NAME"; fi
  - if [[ -f target/release/soundsense-rs.exe ]]; then mv target/release/soundsense-rs.exe "target/release/soundsense-rs-$TRAVIS_TAG-$TRAVIS_OS_NAME.exe"; fi
  
  deploy:
  - provider: releases
    skip_cleanup: true
    on:
      tags: true
      condition: "$TRAVIS_RUST_VERSION = stable"
      branch: release
    file_glob: true
    file:
    - target/release/soundsense-rs-*
    api_key:
      secure: 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
  </code></pre>
  
  <p>error message:</p>
  
  <pre><code>     Running `/home/travis/build/prixt/soundsense-rs/target/release/build/alsa-sys-9e3efef5874a428f/build-script-build`
  
  error: failed to run custom build command for `alsa-sys v0.1.2`
  
  Caused by:
  
    process didn't exit successfully: `/home/travis/build/prixt/soundsense-rs/target/release/build/alsa-sys-9e3efef5874a428f/build-script-build` (exit code: 101)
  
  --- stderr
  
  thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: "`\"pkg-config\" \"--libs\" \"--cflags\" \"alsa\"` did not exit successfully: exit code: 1\n--- stderr\nPackage alsa was not found in the pkg-config search path.\nPerhaps you should add the directory containing `alsa.pc\'\nto the PKG_CONFIG_PATH environment variable\nNo package \'alsa\' found\n"', src/libcore/result.rs:999:5
  
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.
  
  warning: build failed, waiting for other jobs to finish...
  
  error: build failed
  
  The command "cargo build --verbose --release" exited with 101.
  </code></pre>
  
  <p>What commands do I need to add to solve this?</p>