Comment (by Wolfgang Kastaun):

I confirm that the problem is HDF5 version 1.12.0. I was not using the bundled ET HDF5 but the one installed on the OS (Arch Linux). On the HDF groups website this version is announced as major release that breaks backwards compatibility.

Adding the suggested CPPFLAGS did not solve the problem (the rest of the optionlist is identical to ET’s generic.cfg). I note that the flags suggested on the HDF webpage

https://www.hdfgroup.org/2020/03/release-of-hdf5-1-12-0-newsletter-172/

are different from the ones suggested above, but I did not try those yet.

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2572/build-failure-in-pittnullcode