This project is built using LLVM 10.
The Makefile tries to call llvm-config, but this is ambiguous and may not work in some situations (since there are more versions of LLVM than just v10). The version 10-specific llvm-config binary is often called llvm-config-10; adjust the Makefile if this is the case for your distribution.
See https://apt.llvm.org/ for information. Briefly:
wget --no-check-certificate -O - https://apt.llvm.org/llvm-snapshot.gpg.key \
| sudo apt-key add -
sudo add-apt-repository 'deb http://apt.llvm.org/bionic/ llvm-toolchain-bionic-10 main'
sudo apt update
audo apt install llvm-10-dev
The last command may fail when looking for libFFI 6, in which case you will have to install this dependency manually from Ubuntu 19.04's pacakges (available here).