implement stdlib Jinja function

[h-vetinari]

The next release of conda-build is going to contain a new function to allow expressing constraints on the c standard library version (and implementation).

We plan to use this extensively in conda-forge, because it is the key to handle packages that require newer glibc or Macos versions than our global baseline (in fact, we cannot properly move our baseline on Macos unless we have this).

Since boa is still used used widely in the build infrastructure of conda-forge, it would be essential to have the same functionality also in boa. I had briefly talked about this with @wolfv, and the implementation shouldn't be hard. It definitely wasn't a big deal on the conda-build side (most of that PR is docs and tests).

[isuruf]

Since boa is still used used widely in the build infrastructure of conda-forge, it would be essential to have the same functionality also in boa.

Boa is not used in conda-forge to build recipes. Only conda-mambabuild which is a wrapper around conda-build to use mamba.

It's good to have this function in boa recipes in either case.