Use stack's nix support

[nmattia]

This makes use of stack --nix instead of providing tools/userchroot. It provides a shell.nix that can be used but now the users can build with stack --nix build. For me stack --nix test works out of the box. The nix support is disabled by default in the stack.yaml, hence you do need to pass --nix to your stack commands.

This fixes a few issues I ran into while trying to build:

• The nixpkgs version is pinned, with a script nix/update that will update the commit to the header of release-17.09.
• The tensorflow library used is patched to ensure the libraries (libc, libstdc++) are present on the system.

AFAICT this removes the need for buildFHSUserEnv.

Now I need to figure out what that Google CLA is about...

[tensorflow-jenkins]

Can one of the admins verify this patch?

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[blackgnezdo]

Thanks for the patch. Please sign CLA.

@grwlf who added nix support in the first place should probably take a look as I don't understand nor use Nix.

[nmattia]

I signed it!

[googlebot]

CLAs look good, thanks!

[blackgnezdo]

Jenkins test this please.

[traleig1]

This works fantastically, thanks

[jyp]

What is blocking this to be merged? (Apart from the obvious conflict?) @grwlf ?

[fkm3]

I think we just want someone else nix literate to take a look. If you'd like to look over the code and give a thumbs up that should be good enough :)

[jyp]

This PR is a definite improvement over what's currently in master. So I recommend merging it.
It needs to be rebased though. Mainly the version of tensorflow-c should be updated so that it would match the proto. Perhaps @nmattia can confirm.

[fkm3]

Thanks! @nmattia do you mind rebasing? (I think you just need to update the version in the tarball URL + its SHA)

[nmattia]

Yep, will do. I should have a few free cycles tomorrow.

[nmattia]

I've:

• Updated to tensorflow 1.8.0
• Added stack to the nix-shell (so that you don't even need stack installed)
• Updated nixpkgs
• Set GHC to 8.2.2 (the latest in nixpkgs is > 8.2.2 which doesn't work with the LTS)
• Rebased

@jyp can you let me know if this still works for you? I don't have a global stack install, and adding stack to the nix-shell may cause issues there.

[jyp]

@nmattia Thanks a lot; this is a very useful PR. (I don't have a global stack install either.)

[blackgnezdo]

This seems like an overall improvement, so let's merge it.