Mark destructor of DebugStream as noexcept(false)

C++11 defaults destructors to noexcept.
Good catch, GCC 6!

Adjusted to check for availability of noexcept specifier, as that is not
a given in 2.4.

(cherry picked from commit 45797c46)
Signed-off-by: Steffen Müthing <muething@dune-project.org>