Xcode/Swift universal binary problem

I am using Macaw vector graphics and need to convert my app from x86_64 architecture to a universal x86/amd64 architecture. Macaw won't build for amd64 so I would like to conditionally build that library only for x86 and ignore amd64. How can I do this? I tried protecting blocks of code with #if arch(x86_64) but I get errors that the library isn't found then.