I'm using the HP ANSI C compiler on HP-UX 11 and it doesn't like unnamed unions. The only place in the whole GTK+ 3 source code that uses them is the union definition inside struct _GtkSymbolicColor (around line 60 of gtk/gtksymboliccolor.c).

Unfortunately, not only do you have to name the union (e.g. sym_col) like this at the end of the struct definition:

    struct
    {
      gchar *theme_class;
      gint id;
    } win32;
  } sym_col;
};

but HP's C also then wants you to use the name to access the union as well :-(
Examples with the union name added:

Line 100:       g_free (color->sym_col.name);
Line 103:       gtk_symbolic_color_unref (color->sym_col.shade.color);
and so on.

If no-one wants to edit the 50+ lines involved, I could do it agianst gtk+ 3.6.1 and post up a context diff here as a starting poiint.