Fix sysroot issue about CONDA_BUILD_SYSROOT

[katietz]

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.

This issue fixes that if the environment variable CONDA_BUILD_SYSROOT is defined, the '-sysroot' option passed as argument is ignored and even leads to an unused-command-line-argument warning for it.
This issue is especially problematic in cross-compiler scenario - like emscripten - where it is essential to be able to switch between native and cross' sysroot by the '-sysroot' option on demand.

As this issue is a long standing one, we might even want to backport it to older versions, but this is for now out of scope for this patch

[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 (recipe) and found it was in an excellent condition.

[isuruf]

AFAIK, this was an intentional change that CONDA_BUILD_SYSROOT would override isysroot. Some software like Qt was using passing wrong -isysroot on Darwin.
However I think this is a good change even though it is backwards incompatible.

[katietz]

yeah, I think such issues should be better fixed in venture's build-system instead

[h-vetinari]

Fine by me. Do we merge?

Also, how far back do you reasonably need the backports? I could do 14.x & 15.x.

[katietz]

well, I think those both versions should be fine