faust2: fix build

Fixes https://github.com/NixOS/nixpkgs/issues/12749

The build failure was caused by brittle detection of the
llvm version. See the code for (excessive) details. This fix
is a quick hack, a proper fix would be to parse the version
of the input llvm derivation and use that to derive a proper
value. Here we just pin the version.

Also move build-time deps to `nativeBuildInputs`.