It is recommended that you install llvm source and the appropriate llvm-gcc binaries, rather than using apt-get or similar software. Proteus has been developed against version 2.7 of LLVM (documentation download). There are later versions of LLVM but Proteus has not been fully tested against later versions. Although in principle they should work, however there is the risk that new features will cause some issues. Please report any problems that you find with these?
Documentation links to LLVM will be provided to the 2.7 version.
Downloading from the website the llvm source and the installing llvm-gcc frontend binaries has proved to be a straightforward process as outlined in the Getting Started documentation. One point to note is that the prebuilt binaries are not compiled with fortran support and if you wish to use that you will need to rebuild them for your platform.
A basic introduction with using LLVM is given here