TruffleRuby needs LLVM to run and build C extensions - version 3.8, 3.9 or 4.0.
For using C extensions:
apt-get install libc++-dev libc++abi1
Note that we install libc++-dev here, as installing libc++ seems to
introduce some system conflicts.
Additionally, for building C extensions:
apt-get install clang llvm libc++abi-dev
We need the opt command, so you can't just use what is installed by Xcode if
you are on macOS. We would recommend that you install LLVM via
Homebrew and then manually set your path.
brew install llvm
export PATH="/usr/local/opt/llvm/bin:$PATH"