Suppress deprecated warnings for g_mutex_new() and g_mutex_free()

    milter-client.c: In function '_milter_client_init':
    milter-client.c:477:5: warning: 'g_mutex_new' is deprecated (declared at /usr/include/glib-2.0/glib/deprecated/gthread.h:272) [-Wdeprecated-declarations]
         priv->quit_mutex = g_mutex_new();
         ^
    milter-client.c: In function 'dispose':
    milter-client.c:758:9: warning: 'g_mutex_free' is deprecated (declared at /usr/include/glib-2.0/glib/deprecated/gthread.h:274) [-Wdeprecated-declarations]
             g_mutex_free(priv->quit_mutex);
             ^

GitHub: see #32
Suppress deprecated warnings for g_thread_create()

    milter-client.c: In function 'single_thread_start_accept':
    milter-client.c:1486:5: warning: 'g_thread_create' is deprecated (declared at /usr/include/glib-2.0/glib/deprecated/gthread.h:100): Use 'g_thread_new' instead [-Wdeprecated-declarations]
         thread = g_thread_create(single_thread_accept_thread, client, TRUE,
         ^

GitHub: see #32