title: Cabal: can't install gloss-raster because of LLVM
body: <p><code>cabal install gloss-raster</code> outputs:</p>
  
  <pre><code>Resolving dependencies...
  Configuring gloss-raster-1.7.4.3...
  Building gloss-raster-1.7.4.3...
  Preprocessing library gloss-raster-1.7.4.3...
  [1 of 2] Compiling Graphics.Gloss.Raster.Array ( Graphics/Gloss/Raster/Array.hs, dist/build/Graphics/Gloss/Raster/Array.o )
  Warning: Couldn't figure out LLVM version!
           Make sure you have installed LLVM
  ghc: could not execute: opt-3.0
  cabal: Error: some packages failed to install:
  gloss-raster-1.7.4.3 failed during the building phase. The exception was:
  ExitFailure 1
  </code></pre>
  
  <p>I've installed LLVM (<code>apt-get install llvm</code>) and its Haskell libraries (<code>cabal install llvm</code>), so what's wrong with LLVM?</p>
  
  <p>I tried installing a specific version with <code>sudo cabal install llvm-3.0</code> an I got:</p>
  
  <pre><code>Resolving dependencies...
  cabal: Could not resolve dependencies:
  next goal: llvm (user goal)
  rejecting: llvm-3.0.1.0/installed-d41..., 3.0.1.0, 3.0.0.0, 0.10.0.1,
  0.10.0.0, 0.9.1.2, 0.9.1.1, 0.9.1.0, 0.9.0.1, 0.8.2.0, 0.8.1.0, 0.8.0.2,
  0.7.1.2, 0.7.1.1, 0.7.1.0, 0.7.0.1, 0.7.0.0, 0.6.8.0, 0.6.7.0, 0.6.6.0,
  0.6.5.0, 0.6.4.0, 0.6.3.0, 0.6.2.0, 0.6.0.3, 0.6.0.2, 0.5.0.1, 0.4.4.2,
  0.4.4.1, 0.4.2.0, 0.4.1.0, 0.4.0.3, 0.4.0.1, 0.4.0.0, 0.0.2 (global constraint
  requires ==3.0)
  </code></pre>