Github user greghogan commented on the issue:
https://github.com/apache/flink/pull/4367
License isn't an issue since it's only used during the build:
CAN APACHE PROJECTS DISTRIBUTE COMPONENTS UNDER PROHIBITED LICENSES?
https://www.apache.org/legal/resolved.html#prohibited
Apache projects cannot distribute any such components. This means that no
source code can be from Category X and that any convenience binaries produced
may not include such contents. As with the previous question on platforms, the
component can be relied on if the component's license terms do not affect the
Apache product's licensing. For example, using a GPL'ed tool during the build
is OK, however including GPL'ed source code is not.
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