get source code via git clone https://github.com/autofac/Autofac
open then solution Autofac.sln and build.
However, if you look at Reference of Autofac.xproj, it has two References, one is .net 4.5 and another one is .net core 1.0.
the output dll will be two as well, in bin folder, you could see two folders with name of two versions, one dll is for .net 4.5, another one is for .net core 1.0. (very important).
if you add some code (for example, System.Diagnostics.Trace.WriteLine) only supported by .net 4.5, then you could build .net 4.5 dll only.