which executes pylicense with all configs matching
"PROJECT_ROOT/.pylicense*.cfg"

* common.so -> _common.so, to allow for magic in __init__.py
* create module infrastructure in build tree by symlink during cmake

- don't know exactly why it works like this, I suspect it is
due to the DEPENDENCYCHECK dune/xt/common/test/CMakeLists.txt
that depends on the xtcommon variable defined in the header
listing

see http://clang.llvm.org/extra/clang-tidy/

`add_format` now only takes one argument: a directory that is used
the base dir for recursive globbing files into clang-format
Usages of the macros are adjusted accordingly. It's still possible to
use it multiple times in a project, if one for whatever reason
does not wnat to throw it at the source root. Only one `format`
target will be generated either way, with one subtarget per
`add_format` invocation.