m4: Use -Wdeprecated-declarations
    
    Let's enable deprecated-declarations warnings as we want to:
    1) Avoid insert/maintain deprecated widgets/methods
    2) Avoid adding widgets/methods that are too new, what could cause
    problems like virt-viewer not being able to build in a specific distro.
    
    Patches for making these two items possible are coming, introducing
    _VERSION_MAX_ALLOWED for both GLIB and GDK and removing (as much as
    possible) deprecated widgets/methods/structures.
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    Acked-by: Pavel Grunt <pgrunt@redhat.com>