[cmake] improvement of CMakeLists.txt

dune/fem-dg/CMakeLists.txt

dune/fem-dg/assemble/CMakeLists.txt

dune/fem-dg/main/CMakeLists.txt

dune/fem-dg/misc/CMakeLists.txt

dune/fem-dg/models/CMakeLists.txt

dune/fem-dg/operator/CMakeLists.txt

dune/fem-dg/operator/adaptation/CMakeLists.txt

dune/fem-dg/operator/dg/CMakeLists.txt

dune/fem-dg/operator/fluxes/CMakeLists.txt

dune/fem-dg/operator/limiter/CMakeLists.txt

dune/fem-dg/pass/CMakeLists.txt

dune/fem-dg/solver/CMakeLists.txt

dune/fem-dg/stepper/CMakeLists.txt

dune/fem-dg/test/CMakeLists.txt

dune/fem-dg/test/advdiff/CMakeLists.txt

dune/fem-dg/test/euler/CMakeLists.txt

dune/fem-dg/test/navierstokes/CMakeLists.txt

dune/fem-dg/test/poisson/CMakeLists.txt