我正在尝试安装repa-v4l2软件包，我收到错误，我不知道如何解决：
kyle@Kyles-Comp:~$ sudo cabal install repa-v4l2
[sudo] password for kyle:
Resolving dependencies...
Configuring repa-v4l2-0.2.0.0...
Building repa-v4l2-0.2.0.0...
Preprocessing library repa-v4l2-0.2.0.0...
[1 of 2] Compiling Graphics.Webcam.Linux.Internal( src/Graphics/Webcam/Linux/Internal.hs, dist/build/Graphics/Webcam/Linux/Internal.o )
<no location info>:
Warning: Couldn't figure out LLVM version!
Make sure you have installed LLVM
ghc: could not execute: opt
Failed to install repa-v4l2-0.2.0.0
cabal: Error: some packages failed to install:
repa-v4l2-0.2.0.0 failed during the building phase. The exception was:
ExitFailure 1
你如何帮助LLVM找出问题？