Passing multiple files to dune_symlink_to_source_files() was broken.
While at it, I switched the given macros to be functions and have
named arguments. Using positional arguments will result in a warning.

(cherry picked from commit 797fbfa1)
Signed-off-by: Carsten Gräser <graeser@dune-project.org>

I was bitten again by not properly cleaning build directories
while testing build system features. With this patch, the feature
* builds the symlinks in finalize_project instead of dune_project
  Only there the existence of the subdirectories in the build tree
  is guaranteed.
* The globbing expression is sane again.
* Properly excludes build directories
* works with cmake 2.8 and 3.1, per-module or global out-of-source builds.

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.