With this change gdk-mbstowcs actually works.

        * gdk-support.c (gdk_property_change_list, gdk_property_change_vector,
        gdk_mbstowcs_interp, gdk_colormap_alloc_colors_interp): Use g_new
        rather than scm_must_malloc for temporary memory, since it's not
        subject to gc the way scm_must_malloc is meant to express, and
        negatives don't work with scm_done_malloc in Guile 1.6.4.
        gdk_property_change_list and gdk_property_change_vector were in fact
        missing scm_done_malloc, making scm_mallocated increase indefinitely.