g_steal_pointer() as provided by glib improved significantly. Nowadays it
casts the return type via the non-standard typeof() operator.

But this useful feature is only enabled with

    GLIB_VERSION_MAX_ALLOWED >= GLIB_VERSION_2_58

which NetworkManager does not set.

This macro is hardly rocket science. Always provide our own
implementation, that always does the casting (we rely on gcc/clang
to support typeof() already at many places).