Support for building old versions from source

Pre-1.0 versions of Sphinx couldn't previously be built from source, due to Sphinx being inconsistent about their file-naming conventions and URLs.  This PR fixes this, and allows pre-1.0, as well as beta versions, to be built from source.  As part of this, it moves the calculated configure flags and download URL out of the attributes file and into the source recipe, so that overridden `version` and `use_*` attributes are properly applied to the recipe.