g_steal_pointer() is marked as GLIB_AVAILABLE_STATIC_INLINE_IN_2_44,
that means we get a deprecated warning. Avoid that. We anyway
re-implement the macro so that we can use it before 2.44 and so
that it always does the typeof() cast.

(cherry picked from commit edfe9fa9)
(cherry picked from commit 6936a061)
(cherry picked from commit e333a28b)