Replacing a compiler by another make qmake fail

With hello.pro

SOURCES += hello.cpp

and /usr/bin/g++++ pointing to clang-6.0 instead of gcc-7

it is failing with:

$ qmake hello.pro
Project ERROR: failed to parse default search paths from compiler output

It used to work (don't know in which version).

This is causing 223 packages to fail to build in Debian.