With this change, java packages will build with openjdk by default. The
primary driver for this is legal: The build farm is not allowed to
distribute the proprietary Oracle jdk6, and so it is not allowed to
distribute any packages that depend on it. In my view, this is a purely
beneficial change: from the perspective of the build farm, packages will
go from undistributable due to licensing to either distributable or
undistributable due to failed build (if the package doesn't build
properly with openjdk), and from the perspective of the end user it is
very easy to override the jdk on a package-by-package basis or for all
of nixpkgs in the nixpkgs configuration.
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