Fix printing of DEBUG message since glib 2.32

Fix printing of DEBUG message since glib 2.32

Viking uses g_debug function. But in glib 2.32, the g_debug behavior changed and all
DEBUG messages are discarded, unless G_MESSAGES_DEBUG is set. So, "viking -d"
no more work as its design was to mute DEBUG messages when "-d" is not set.

Now, printing is restored via a dedicated function.