[cmake] fixes errors with optional TBB missing

cmake/modules/DuneUtils.cmake

@@ -137,7 +137,7 @@ macro(END_TESTCASES)
         add_dune_alugrid_flags(${test})
         add_dune_superlu_flags(${test})
       endif(dune-grid_FOUND)
-      if(COMMAND add_dune_tbb_flags)
+      if(COMMAND add_dune_tbb_flags AND TBB_FOUND)
         add_dune_tbb_flags(${test})
       endif()
      endforeach( test ${testnames} )

dune/stuff/CMakeLists.txt

@@ -24,7 +24,9 @@ set(lib_dune_stuff_sources common/filesystem.cc
 dune_add_library("dunestuff" ${lib_dune_stuff_sources}
   ADD_LIBS ${DUNE_LIBS})
 add_dune_mpi_flags(dunestuff)
-add_dune_tbb_flags(dunestuff)
+if(TBB_FOUND)
+  add_dune_tbb_flags(dunestuff)
+endif(TBB_FOUND)
 
 if(dune-grid_FOUND)
   add_dune_superlu_flags(dunestuff)