add clangxx for vecmathlib

[matthiasdiener]

Fixes #73.

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.

[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.

[matthiasdiener]

This does appear to address the issue, but not sure if the approach is correct. @isuruf @inducer

[inducer]

I think pocl no longer uses vecmathlib? Aren't they using SLEEF now?