Todd Gamblin fe0fdf60b4 Add ability to prefer particular versions in packages. ... - Adding `preferred=True` to a version directive will change its sort order in concretization. - This provides us a rudimentary ability to keep the Spack stack stable as new versions are added. - Having multiple stacks will come next, but this at least allows us to specify default versions of things instead of always taking the newest.		2015-12-21 15:35:47 -08:00
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spack	Add ability to prefer particular versions in packages.	2015-12-21 15:35:47 -08:00