Comment # 3 on bug 1189991 from Fabian Vogt

resources_big is incompatible with LTO objects, see
https://bugreports.qt.io/browse/QTBUG-73834.

The reason it works with plain "qmake-qt5" instead of the "%qmake5" macro is
that the latter does not set the compile flags to include %optflags, which
contains %_lto_cflags ("-flto=auto").

A workaround is to disable either LTO or resources_big for the affected
package, but it should also be possible to combine both, by letting qmake
handle LTO instead of forcing it through optflags. That way the objects
containing resource data are build without LTO. You could try something like
this:

%define _lto_cflags %{nil}
%qmake5 CONFIG+=ltcg