Invalid symbol in _backend_gdk.c if numpy version < 1.7

[ianthomas23]

PR #3665 added the following to _backend_gdk.c (lines 40-43):

array = (PyArrayObject*)
        PyArray_New(&PyArray_Type, 3, dims, NPY_UBYTE, strides,
                    (void*)gdk_pixbuf_get_pixels(gdk_pixbuf), 1,
                    NPY_ARRAY_WRITEABLE, NULL);

The flag NPY_ARRAY_WRITEABLE is not valid for numpy versions < 1.7, the equivalent flag is NPY_WRITEABLE.

[tacaswell]

Closed by #3725