Fixes cmake: include 'variables.cmake' before declaring project()

Moves the code corresponding to 64-bit OS into CMakeLists.txt,
because CMake sets some important variables (CMAKE_SIZEOF_VOID_P)
after the declaration of the project.

Issue: #50
Adds script FindLLVM.cmake

Replaces CMake variable 'LLVM' with 'USE_LLVM'.
Creates options: USE_LLVM, USE_READLINE.
Run `cmake .. -DUSE_LLVM=ON` to build llst with LLVM.

Issue: #50