Make example cmake targets optional

Alexander Müllerrequested to merge
alexander.mueller/dune-fufem:feature/examples_optional into master

In our CI the following comand for dune-fufem

dune-common/bin/dunecontrol   --cmake-opts="-DCMAKE_CXX_FLAGS=-O3 -DCMAKE_C_COMPILER='gcc-12' -DCMAKE_CXX_COMPILER='g++-12' -DPython3_EXECUTABLE=/usr/bin/python -DPython3_INCLUDE_DIRS=/usr/include/python3.11 DDUNE_PYTHON_ALLOW_GET_PIP=1 -DDUNE_ENABLE_PYTHONBINDINGS=TRUE -DBUILD_SHARED_LIBS=TRUE -DCMAKE_BUILD_TYPE=Release -DCMAKE_CXX_STANDARD=20" cmake &&   dune-common/bin/dunecontrol make

fails with

Building CXX object examples/CMakeFiles/biharmonic.dir/biharmonic.cc.o
446.7 /dune/dune-fufem/examples/biharmonic.cc: In function 'int main(int, char**)':
446.7 /dune/dune-fufem/examples/biharmonic.cc:219:39: error: 'makeAdolCFunction' is not a member of 'Dune::Fufem'
446.7   219 |   auto dirichletValues = Dune::Fufem::makeAdolCFunction(SignatureTag(), dirichletValuesC0);

since we do not have ADOLC installed. This MR let us deactivate the compilation of the examples.

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