Allow enabling dune_symlink_to_source_tree from the opts file

Patching the dune core modules just to have the advantages of
symlinks to the source tree is a bad idea. The patch introduces
a check for the variable DUNE_SYMLINK_TO_SOURCE_TREE instead, which
enables the macro if set.

cmake/modules/DuneMacros.cmake

@@ -670,6 +670,10 @@ macro(dune_project)
     exec_program(chmod ARGS "+x ${CMAKE_BINARY_DIR}/compiler.sh")
     set(CMAKE_CXX_COMPILER ${CMAKE_BINARY_DIR}/compiler.sh)
   endif()
+
+  if(DUNE_SYMLINK_TO_SOURCE_TREE)
+    dune_symlink_to_source_tree()
+  endif()
 endmacro(dune_project)
 
 # create a new config.h file and overwrite the existing one