Disable -Werror,-Wundef-prefix=TARGET_OS as they are not yet defined in the 10.9 SDK used for osx-64

[xhochy]

Fixes #116

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.

[isuruf]

We need to at least keep the warning for TARGET_OS_OSX

[xhochy]

TARGET_OS_OSX

Why? This is neither defined in my local copy of the 10.9 SDK.

[isuruf]

This is neither defined in my local copy of the 10.9 SDK.

Sure, but if some package is depending on TARGET_OS_OSX, we shouldn't ignore the warning. I think that's the whole reason for adding this warning.