Bugfix: Fix all_header tests

The path for include files under ./deal.II/base/std_cxx1x/ was not
constructed correctly. Do the same path mangling as in the case of
./deal.II/base/std_cxx11/
Is it correct that by writing
```
GET_FILENAME_COMPONENT(_path ${_full_file} PATH)
GET_FILENAME_COMPONENT(_path ${_path} NAME)
```
_path simply refers to the *last* component of the path? And that you are prepending `base/` if the last component of path is either std_cxx1x or std_cxx11?