Enable thread local storage when building

[cmcaine]

Fixes #5

[cmcaine]

This works for me. I wrote some tests too, but I can't get them to reliably fail on the old build of Nauty, so 🤷

Here's the test:

using LightGraphs
using Nauty
using Test

function dowork()
    g = LightGraphs.barabasi_albert(63, 5)
    Nauty.baked_canonical_form(g).canong
end

[fetch(Threads.@spawn dowork()) for _ in 1:10000]

# This line won't be executed if something above segfaults.
@test true

The idea is that you run that from the existing test file with:

using PerformanceTestTools: @include_foreach

@testset "Threading tests" begin
    # This starts a new Julia process with 10 threads, includes the test file
    # and checks that its exit code is zero (success).
    @include_foreach("threading-test.jl", [["JULIA_NUM_THREADS" => "10"]])
end

[cmcaine]

I made some silly mistakes when testing the threading code. These new commits include working tests which should reliably fail on builds without thread local storage.

The benchmark is 10-15% slower on my system when thread local storage is enabled.