add gpu support

[izahn]

Checklist

• Used a personal fork of the feedstock to propose changes
• Bumped the build number (if the version is unchanged)
• Reset the build number to 0 (if the version changed)
• Re-rendered with the latest conda-smithy (Use the phrase @conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• Ensured the license file is being packaged.

Fixes #26

[conda-forge-linter]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

I do have some suggestions for making it better though...

For recipe:

• It looks like the 'libxgboost' output doesn't have any tests.
• It looks like the '_py-xgboost-mutex' output doesn't have any tests.
• It looks like the 'py-xgboost-cpu' output doesn't have any tests.
• It looks like the 'xgboost' output doesn't have any tests.
• It looks like the '_r-xgboost-mutex' output doesn't have any tests.
• It looks like the 'r-xgboost-cpu' output doesn't have any tests.

[xhochy]

Can you remind me how we get the cpu package to be the default on installation?

[izahn]

I think this is basically working now, but I could use some help making sure the variants are coordinated correctly. If you can take a look at the build strings and mutex packages and see if it makes sense that would be great!

[trivialfis]

Excited to see progress on GPU build. I'm not familiar with conda packaging, but feel free to ping me if there's anything I can help.

[jakirkham]

Thanks Ista! 😀

Also cc-ing @hcho3

[conda-forge-linter]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

I do have some suggestions for making it better though...

For recipe:

• It looks like the 'libxgboost' output doesn't have any tests.
• It looks like the '_py-xgboost-mutex' output doesn't have any tests.
• It looks like the 'py-xgboost-cpu' output doesn't have any tests.
• It looks like the 'py-xgboost-gpu' output doesn't have any tests.
• It looks like the 'xgboost' output doesn't have any tests.
• It looks like the '_r-xgboost-mutex' output doesn't have any tests.
• It looks like the 'r-xgboost-cpu' output doesn't have any tests.
• It looks like the 'r-xgboost-gpu' output doesn't have any tests.

[izahn]

Can you remind me how we get the cpu package to be the default on installation?

From what I've been able to gather this is done by depending on __cuda which will cause the cpu version to be selected by default on systems without a suitable gpu. I added this in 7742987 that should do the trick.

[izahn]

OK I've cleaned everything up and done my best with mutex and meta packages, and the build strings should all make sense. This is ready for review.

[xhochy]

Thanks @izahn !