Add C package xsel recipe

[ehfd]

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[conda-forge-webservices]

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 (recipes/xsel) and found it was in an excellent condition.

[ehfd]

@conda-forge/help-c-cpp Ready for review.

This package interfaces with the X11 clipboard API.

Since this is not an official X.Org package, keeping Unix-only for now.

[ehfd]

@hmaarrfk
I have seven X11-related packages waiting to be merged. Could you possibly help out?