qmake tries to create cache file in root directory

1. Check out qt5 and its submodule qtbase.
   I have d85ff1e93425a0782cc3a34d018a0b1f21d82a2b and a120d11cb5506ef0a5535e790f23d49595fb8857 respectively.
2. Configure Qt for a developer build.
   My command line:

   /tmp/qt-bisect/build-qt.tmp$ /tmp/qt-bisect/qt5/configure -developer-build -opensource -confirm-license -nomake examples -nomake tests
   

3. Build Qt.

   cmake --build . --parallel
   

4. Use this Qt build for building a project:

   qtrob@rob-desktop:/tmp/qt-bisect/build-test.tmp$ /tmp/qt-bisect/build-qt.tmp/qtbase/bin/qmake /tmp/qt-bisect/qt5/qtbase/examples/gui/analogclock/analogclock.pro
   

   qmake will complain about missing permissions:

   Info: creating stash file /.qmake.stash
   /tmp/qt-bisect/qt5/qtbase/mkspecs/features/toolchain.prf:73: Cannot write cache file /.qmake.stash: Permission denied
   Info: creating stash file /.qmake.stash
   /tmp/qt-bisect/qt5/qtbase/mkspecs/features/toolchain.prf:73: Cannot write cache file /.qmake.stash: Permission denied
   Info: creating stash file /.qmake.stash
   /tmp/qt-bisect/qt5/qtbase/mkspecs/features/toolchain.prf:73: Cannot write cache file /.qmake.stash: Permission denied
   Info: creating stash file /.qmake.stash
   /tmp/qt-bisect/qt5/qtbase/mkspecs/features/toolchain.prf:73: Cannot write cache file /.qmake.stash: Permission denied
   Info: creating stash file /.qmake.stash
   /tmp/qt-bisect/qt5/qtbase/mkspecs/features/toolchain.prf:76: Cannot write cache file /.qmake.stash: Permission denied
   Info: creating stash file /.qmake.stash
   /tmp/qt-bisect/qt5/qtbase/mkspecs/features/toolchain.prf:421: Cannot write cache file /.qmake.stash: Permission denied
   Info: creating stash file /.qmake.stash
   /tmp/qt-bisect/qt5/qtbase/mkspecs/features/toolchain.prf:423: Cannot write cache file /.qmake.stash: Permission denied
   

Trying to write to / seems fundamentally wrong.