Since this is a package uses C++, it will require the package to be compiled. This is automatically done for you by Atom, but it does have some requirements. LibClang is detected via llvm-config command. It needs to build and link against the LibClang headers and library. It attempts to find the newest version of llvm-config it can on your system. One can override the search by having the LLVM_CONFIG environment variable set, e.g. LLVM_CONFIG=/opt/clang+llvm-3.9.0-x86_64-linux-gnu-ubuntu-16.04/bin/llvm-config. Currently, I do not know of way "at package install time" through Atom to let the user specify which one to use. See llvm-config.sh.