Allow declaring possible values for variants with an associated condition. If the variant takes one of those values, the condition is imposed as a further constraint.
The idea of this PR is to implement part of the mechanisms needed for modeling [packages with multiple build-systems]( https://github.com/spack/seps/pull/3). After this PR the build-system directive can be implemented as:
```python
variant(
'build-system',
default='cmake',
values=(
'autotools',
conditional('cmake', when='@X.Y:')
),
description='...',
)
```
Modifications:
- [x] Allow conditional possible values in variants
- [x] Add a unit-test for the feature
- [x] Add documentation
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