Fixes#4112
This commit extends the support of the AutotoolsPackage methods
`with_or_without` and `enable_or_disable` to bool-valued variants. It
also defines for those functions a convenience short-cut if the
activation parameter is the prefix of a spec (like in
`--with-{pkg}={prefix}`).
This commit also includes:
* Updates to viennarna and adios accordingly: they have been modified to
use `enable_or_disable` and `with_or_without`
* Improved docstrings in `autotools.py`. Raise `KeyError` if name is
not a variant.
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