* boost: do not access the context-impl variant for versions below 1.65.0
* boost: check if spec.variants contains context-impl
* boost: improve error message when the context-impl variant causes a conflict
Executing spack solve boost@1.63.0 +context context-impl=fcontext
triggers the following error message:
==> Error: No version for 'boost' satisfies '@1.63.0' and '@1.79.0'
With this change, the error message becomes the following:
==> Error: Cannot set variant 'context-impl' for package 'boost' because the variant condition cannot be satisfied for the given spec
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