install fails due to missing backend/istl/geneo/conbase_exterior.hh

Steps to reproduce:

• clone dune-pdelab and dependent modules into a common directory, e.g. /tmp/dune-test
• dune-common/bin/dunecontrol exec git checkout -b releases/2.6 (see #111 (comment 44281))
• echo 'CMAKE_FLAGS="-DCMAKE_INSTALL_PREFIX=/tmp/dune-test/install"' >opts
• dune-common/bin/dunecontrol --opts=opts configure : make -j4 install

Relevant log snippet (full log):

-- Installing: /tmp/dune-test/install/include/dune/pdelab/backend/istl/geneo/coarsespace.hh
CMake Error at dune/pdelab/backend/istl/geneo/cmake_install.cmake:36 (file):
  file INSTALL cannot find
  "/tmp/dune-test/dune-pdelab/dune/pdelab/backend/istl/geneo/conbase_exterior.hh".
Call Stack (most recent call first):
  dune/pdelab/backend/istl/cmake_install.cmake:64 (include)
  dune/pdelab/backend/cmake_install.cmake:49 (include)
  dune/pdelab/cmake_install.cmake:38 (include)
  cmake_install.cmake:88 (include)


Makefile:115: die Regel für Ziel „install“ scheiterte
make: *** [install] Fehler 1
--- Failed to build dune-pdelab ---
Terminating dunecontrol due to previous errors!

(Discovered by one of our students in Münster)