Add {{ stdlib('c') }} jinja function for recipes

[danpetry]

Destination channel: defaults

Explanation of changes:

• This enables you to put {{ stdlib('c') }} in recipes
• currently it won't do anything, as the sysroot packages are pulled in by the compilers anyway, and the versions are (I assume?) tied together
• It does allow us to reduce a class of diffs when pulling across conda recipes (that is, we don't have to comment out {{ stdlib('c') }})

See https://docs.conda.io/projects/conda-build/en/stable/resources/compiler-tools.html#expressing-the-relation-between-compiler-and-its-standard-library for more details.

[JeanChristopheMorinPerso]

@danpetry I added a link to the docs in the description. (I still have to review the PR).

[Jrice1317]

According the link JC added in the description, there should be the following for osx. Does this apply to us?

- macosx_deployment_target # [osx]

[danpetry]

@Jrice1317 later, yes, but not needed for now.