CMake 3.1 issued a lot of warnings because it tried to symlink:
- git specific subdirs that are not present in the build tree
- build directories that are a subdir of the source instead of a sibling

The change
- adds a symlink iff a CMakeLists file was found in the source dir.
- excludes such build directory.

The file DuneMacros.cmake is very crowded. Macros that belong
together should be placed in a separate module in order to increase
the visibility of its documentation IMO.