Require MacOS SDK 10.13

[akrherz]

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.

I believe this is the way to keep Mac OS SDK at 10.13 until an upstream release supporting 11.0 >

@conda-forge-admin please rerender

[conda-forge-admin]

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/recipe.yaml) and found it was in an excellent condition.

[akrherz]

Second attempt to getting this set right.
@conda-forge-admin please rerender

[akrherz]

That appeared to work 🥳 , now to see if my theory holds and this PR turns green!

[akrherz]

One transient failure for osx_64

curl: (6) Could not resolve host: github.com

Will see if the other fails before restarting.

[akrherz]

Same test failures, phooey, but this message may imply my change is not sufficient, hmmm

ld: warning: dylib ($PREFIX/lib/libc++.dylib) was built for newer macOS version (11.0) than being linked (10.13)

[akrherz]

zeromq/libzmq/pull/4845 looks promising, trying it here...

[akrherz]

no need for my noise given #88 has landed, thanks!