* headers.mk: add new header to distribution.

	* mln/debug/all.hh,
	* mln/debug/essential.hh: include quiet.hh

	* mln/debug/println.hh: use debug::quiet. Println prints nothing if
	debug::quiet is set to true.

	* mln/debug/quiet.hh: the new global variable.

	* tests/unit_test/Makefile.am,
	* tests/unit_test/mln_debug_quiet.cc: new unit test.

git-svn-id: https://svn.lrde.epita.fr/svn/oln/trunk@3172 4aad255d-cdde-0310-9447-f3009e2ae8c0