gEDA-dev: add-on dir name for pcb

[DJ Delorie]

Ok, I checked this in.  The search logic is:

  hid_load_dir (Concat (EXECPREFIXDIR, "/lib/pcb/plugins/", HOST, NULL));
  hid_load_dir (Concat (EXECPREFIXDIR, "/lib/pcb/plugins", NULL));
  home = getenv("HOME");
  if (home)
    {
      hid_load_dir (Concat (home, "/.pcb/plugins/", HOST, NULL));
      hid_load_dir (Concat (home, "/.pcb/plugins", NULL));
    }
  hid_load_dir (Concat ("plugins/", HOST, NULL));
  hid_load_dir (Concat ("plugins", NULL));

Where EXECPREFIXDIR is --exec-prefix (defaults to --prefix), and HOST
is the --host (i686-pc-linux-gnu for me).