[CMake] Add macro dune_symlink_to_source_tree

Add a symlink called src_dir to all directories in the build tree.
That symlink points to the corresponding directory in the source tree.
Call the macro from the toplevel CMakeLists.txt file of your project.
You can also call it from some other directory, creating only symlinks
in that directory and all directories below. A warning is issued on
Windows systems.

cmake/modules/DuneMacros.cmake

@@ -1161,3 +1161,28 @@ macro(add_dune_all_flags targets)
     target_link_libraries(${target} ${DUNE_LIBS} ${libs})
   endforeach()
 endmacro(add_dune_all_flags targets)
+
+# add a symlink called src_dir to all directories in the build tree.
+# That symlink points to the corresponding directory in the source tree.
+# Call the macro from the toplevel CMakeLists.txt file of your project.
+# You can also call it from some other directory, creating only symlinks
+# in that directory and all directories below. A warning is issued on
+# Windows systems.
+macro(dune_symlink_to_source_tree)
+  # check for Windows to issue a warning
+  if(${CMAKE_SYSTEM_NAME} STREQUAL "Windows")
+    if(NOT DEFINED DUNE_WINDOWS_SYMLINK_WARNING)
+      message(WARNING "Your module wanted to create symlinks, but you cannot do that on your platform.")
+      set(DUNE_WINDOWS_SYMLINK_WARNING)
+    endif()
+  else()
+    # get a list of all files in the current source directory and below.
+    file(GLOB_RECURSE files RELATIVE ${CMAKE_SOURCE_DIR} "*")
+
+    # iterate over all files, extract the directory name and write a symlink in the corresponding build directory
+    foreach(f ${files})
+      get_filename_component(dir ${f} DIRECTORY)
+      execute_process(COMMAND ${CMAKE_COMMAND} "-E" "create_symlink" "${CMAKE_SOURCE_DIR}/${dir}" "${CMAKE_BINARY_DIR}/${dir}/src_dir")
+    endforeach()
+  endif()
+endmacro(dune_symlink_to_source_tree)