Peter Scheibel 7b2895109c Document how to add conditional dependencies (#14694) ...

* add short docs section on conditional dependencies
* add reference to spec syntax
* add note that conditional dependencies can save time

2020-01-30 12:34:54 -08:00

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spack

Document how to add conditional dependencies (#14694)

2020-01-30 12:34:54 -08:00