qt5 configuring static build fails, QMAKE_SPEC unset

When trying to configure qt5 (using sources from qt-everywhere-src-5.11.0.tar.xz) with:

./configure -opensource -confirm-license -release -static -nomake examples -nomake tests

for a static build, the configuration fails after building qmake with output:

could not find qmake spec "".

However qmake is built (more or less successfully). Querying the qmake persistent properties with qmake -query yields empty fields for QMAKE_SPEC and QMAKE_XSPEC.

Adding an echo $QMAKESPEC in the qtbase/configure file shows, that at least $QMAKESPEC is set to a sensible value (.../linux-g++).

I tried with qt5.11.0 and qt5.10.1. Which both don't work, albeit the message from qt5.10.1 is a little bit different. I also tried to add -platform and -spec options to ./configure, but this didn't help.

With openSUSE Leap 42.3 (gcc 4.8), qt5.11.0 can be built successfully. Note that I am running the build process in a docker container with the official opensuse images (which however shouldn't matter here.)