qtbase-opensource-src still fails to cross build. When you attempt to
cross build it for ppc64el it fails quite early, because it lacks
mkspecs. For some reason, qt needs these for every single architecture
even though they could be generated automatically. I'm attaching the
patch for ppc64el based on the relevant arm64 files, but I do wonder
whether this could be solved generically somehow. Any ideas?

Helmut

Hi Helmut!
approach for cross-building.

In Debian I was trying to use a different approach for cross-compiling
Qt applications and modules (except qtbase): using a generic mkspec
(linux-g++) and specifying the toolchain in a qt.conf file.

Ideally a similar thing should be used for qtbase. The configure script
does not seem to support passing qt.conf file, but one can pass variables
like QMAKE_CXX=... etc to it.

(Unfortunately I don't have time to look at it now.)