One can now set the variable OPM_BINARY_PACKAGE_VERSION to a
meaningful version string (Debian 11.2: 2021.10-4). When building
outside of git it will be used as the project hash. If the variable is
non-empty there will also be no timestamp in project-timestamp.hh (as
this would break reproducible builds).
Note that if building from git or not setting the variable then everything
will be like before.
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