Merge pull request #4699 from ethereum/cpp-build-env

cpp-build-env: Install LLVM dev package

Author: chfast

scripts/docker/cpp-build-env.dockerfile

@@ -34,6 +34,7 @@ RUN export DEBIAN_FRONTEND=noninteractive \
     # g++-7 \
     clang-5.0 \
     llvm-5.0 \
+    llvm-5.0-dev \
     # Dependencies
     libleveldb-dev \
   && rm -rf /var/lib/apt/lists/* \