Comment (by Roland Haas):

ah, that's why. Version 1.89 (I had misremembered 1.69). Should not be able to use that one anyway (at least Steve says versions newer than 1.84 fail).

My self-compiled one was the one in Externallibraries which is 1.84. And indeed the file is last present in Debian's Boost 1.88 package.

Since we cannot (though apparently things worked of for you) use that new version yet, the extra check does no harm.

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/1775/add-boost-to-et