AddThreadsFlags
---------------

Global thread-library integration for DUNE modules.

If the CMake package :cmake:command:`find_package(Threads)` succeeds, this
module:

``HAVE_THREADS``
  Sets the config-header variable to reflect whether the thread library was
  found.

``Threads::Threads``
  Links the imported thread library target globally via
  :cmake:command:`link_libraries()`. As a result, targets created afterwards in
  the current directory scope inherit the thread library automatically.