[WIP] Option to use system MKL instead of MKL_jll

[carstenbauer]

This is a quick first attempt to solve #82 based on the ideas discussed in the context of MPI.jl here: JuliaParallel/MPI.jl#483.

Essentially, I'm trying to add a package preference use_jll to MKL.jl (using Preferences.jl) based on which the package desides to using MKL_jll or setting const libmkl_core and const libmkl_rt explicitly (pointing to a system MKL). The desired workflow is: (1) set JULIA_MKL_USE_JLL=false (2) build MKL -> MKL_jll artifact is never downloaded (it is lazy) and system MKL is used.

Sharing this early attempt (which might turn out as rubbish) to facilitate a discussion with @staticfloat started on Slack.

[carstenbauer]

The find_uuid_in_project business is because @set_preferences! tries to operate on the Main module (this is where build.jl is executed I guess) rathern than MKL.jl. So I use the functions set_preferences!(uuid, ....) directly. Maybe that's complete nonsense and why this isn't working so far.

[carstenbauer]

Draft by @staticfloat: https://gist.github.com/staticfloat/817fd946216a1af37ea4835402bbcbae

Looks like a (much!) nicer approach.

[carstenbauer]

See #84.