This is a tricky one. I was getting the following warnings:

gdkdnd-x11.c: In function `motif_read_target_table':
gdkdnd-x11.c:920: warning: dereferencing type-punned pointer will break
strict-aliasing rules
gdkdnd-x11.c: In function `motif_check_dest':
gdkdnd-x11.c:1270: warning: dereferencing type-punned pointer will break
strict-aliasing rules
gdkdnd-x11.c: In function `motif_read_initiator_info':
gdkdnd-x11.c:1464: warning: dereferencing type-punned pointer will break
strict-aliasing rules
gdkdnd-x11.c: In function `xdnd_check_dest':
gdkdnd-x11.c:2384: warning: dereferencing type-punned pointer will break
strict-aliasing rules
gdkdnd-x11.c:2404: warning: dereferencing type-punned pointer will break
strict-aliasing rules
gdkdnd-x11.c: In function `xdnd_read_actions':
gdkdnd-x11.c:2451: warning: dereferencing type-punned pointer will break
strict-aliasing rules
gdkdnd-x11.c: In function `xdnd_enter_filter':

What I've found is the following:

      MotifTargetTableHeader *header = NULL;
      .
      .
      .
      XGetWindowProperty (display_x11->xdisplay,
                          display_x11->motif_drag_window,
                          motif_drag_targets_atom,
                          0, (sizeof(MotifTargetTableHeader)+3)/4, FALSE,
                          motif_drag_targets_atom,
                          &type, &format, &nitems, &bytes_after,
                          (guchar **)&header);

is breaks gcc's C99 strict aliasing rules. To get around this, you need to
convert a guchar** to a MotifTargetTableHeader*. The only way I can see that is
valid is to create a guchar**, use this in parameter 12 of XGetWindowProperty,
cast this to a temporary gpointer, and then cast the temporary gpointer to an
XGetWindowProperty* variable. 

So the previous fragment will look like:

      MotifTargetTableHeader *header = NULL;
      .
      .
      .
      guchar **pheader;
      gpointer temp_header;
                                                                               
                                                                   .
      .
      .
      XGetWindowProperty (display_x11->xdisplay,
                          display_x11->motif_drag_window,
                          motif_drag_targets_atom,
                          0, (sizeof(MotifTargetTableHeader)+3)/4, FALSE,
                          motif_drag_targets_atom,
                          &type, &format, &nitems, &bytes_after,
                          pheader);

      temp_header = (gpointer) &pheader;
      header = (MotifTargetTableHeader*) &temp_header;

Very much a hack job, I feel (in fact I say as much in a comment in the diff).
However, it does remove the strict-aliasing warnings in gcc.