Build a variant without features to use latest sysroot outside conda-build (redux)

[h-vetinari]

Rebase of #75, since @isuruf said in #87:

This would prevent me from cross compiling for newer sysroots from older sysroots. Correct fix is #75

but hasn't responded to

@h-vetinari: OK cool. Would you mind rebasing that PR, or do you want me to do it?

Also, if you wouldn't mind explaining, I'd be curious what the use-case is for needing an older sysroot in build: than the one for the target.

All but the core commit were obsoleted by changes in the meantime (I didn't squash the others, just dropped them, since not applicable anymore).

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

I do have some suggestions for making it better though...

For recipe/meta.yaml:

• ℹ️ The recipe is not parsable by parser conda-souschef (grayskull). This parser is not currently used by conda-forge, but may be in the future. We are collecting information to see which recipes are compatible with grayskull.
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