Rework build process to allow integration of LLVM tools

[radl97]

This PR contains the first patch aiming to allow usage of LLVM tools (mostly opt) in the project.

Usage of opt would enable single pass tests, or later, even remove most, if not all driver (pass management) code.

It seems to me that if the build process got faster, however, it is partly because the shared libraries are not linked together (instead, the shared lib GazerLLVM.so knows all the modules used in the build process).