it seems that at least some of the ExternalLibraries fail to compile with cmake 4.0 claiming that cmake is too old and that at least cmake 3.5 is required .

This looks like they are doing the version check incorrectly (since there is a bump in the major cmake version).

A workaround is to downgrade cmake or to use the CMake thorns in ExternalLibraries and its embedded cmake.

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2883/externallibraries-fail-to-build-with-cmake