Syntax to specify specific variant in requirements

[GuillaumeQuenneville]

Checklist

• I added a descriptive title
• I searched open reports and couldn't find a duplicate

What happened?

Hi,

I can't seem to find in the documentation how to specify requirements that need a specific variant

  - fftw[build=mpi_openmpi*]
  - hdf5[build=mpi_openmpi*]

Are valid in environment files and in conda install -c conda-forge "fftw[build=mpi_openmpi*]" but not in meta.yaml's. It returns

Error: Invalid selector in meta.yaml line 24:
offending line:
    - fftw[build=mpi_openmpi*]
exception:

Using {{ "fftw[build=mpi_openmpi*]" }} returns the same issue so I don't think it's an issue with jinja substitution.

Thank you

Additional Context

Steps to reproduce

 conda skeleton pypi click
# Add - fftw[build=mpi_openmpi*] to any of the requirements in the meta.yaml
conda build click

WARNING: No numpy version specified in conda_build_config.yaml.  Falling back to default numpy value of 1.22
Error: Invalid selector in meta.yaml line 22:
offending line:
    - fftw[build=mpi_openmpi*]
exception:
invalid syntax (<string>, line 1)

Versions tested

conda                     24.1.0           py39h06a4308_0
conda-build               24.1.0           py39h06a4308_0