title: Why doesn't Stack allow relative paths?
body: <p>I'm working on a library which depends on an external C library, and would like to bundle the headers and lib files with my project. The <code>stack.yaml</code> file requires all paths to be absolute, which is clearly not ideal for this purpose.</p>
  
  <p>As per @adamse's request, here is my <code>stack.yaml</code>:</p>
  
  <pre><code>flags: {}
  
  extra-package-dbs: []
  packages:
  - '.'
  extra-deps:
  - ALUT-2.4.0.2
  - OpenAL-1.7.0.4
  resolver: lts-7.2
  extra-lib-dirs:
  - './dependencies/freealut/admin/VisualStudio6/alut/x64/Release'
  - './dependencies/openal-soft-1.16.0-bin/openal-soft-1.16.0-bin/libs/Win64'
  </code></pre>
  
  <p>and the error message:</p>
  
  <pre><code>Could not parse 'C:\Users\Jonatan\Desktop\Haskell\modules\AbbeyRoad\stack.yaml':
  Aeson exception:
  Error in $['extra-lib-dirs'][0]: failed to parse field 'extra-lib-dirs': InvalidAbsDir "./dependencies/freealut/admin/VisualStudio6/alut/x64/Release"
  See http://docs.haskellstack.org/en/stable/yaml_configuration/
  </code></pre>
  
  <p><code>.cabal</code>:</p>
  
  <pre><code>  extra-libraries:     OpenAL32, alut
    extra-lib-dirs:      ./dependencies/freealut/admin/VisualStudio6/alut/x64/Release, ./dependencies/openal-soft-1.16.0-bin/openal-soft-1.16.0-bin/libs/Win64
  </code></pre>