rtapi/Submakefile:  don't link avahi libs with rtapi_app

`dpkg-shlibdeps` finds it's a useless dependency
docs/src/Submakefile:  silence warning when $MANPATH is set

   /usr/bin/mandb: warning: $MANPATH set, ignoring /etc/manpath.config
xenomai-kernel:  silence -Wdeprecated-declarations compile warning

Xenomai kernel thread modules are deprecated and throw a warning.

    In file included from [...]/xenomai-kernel.h:26:0,
		     from [...]/rtapi_common.h:93,
		     from [...]/rtapi_task.c:33:
    /usr/include/xenomai/native/task.h: In function ‘rt_task_spawn’:
    /usr/include/xenomai/native/task.h:319:5: warning: ‘rt_task_create’
       is deprecated (declared at /usr/include/xenomai/native/task.h:252)
       [-Wdeprecated-declarations]
	 int err = rt_task_create(task,name,stksize,prio,mode);
	 ^
Fixup for ecbf70be