gEDA-dev: add-on dir name for pcb

[DJ Delorie]

> I was originally thinking that they could go in $prefix/share/, but
> I think for consistency with other apps, .so files ought to go in
> $prefix/lib/appname/

Technically, that would be $exec_prefix/...

So...

$exec_prefix/lib/pcb/*.so
or
$exec_prefix/lib/pcb/hids/*.so
or
$exec_prefix/lib/pcb/plugins/*.so  ?

As for $HOME, I think ~/.pcb/hids/ makes the most sense; we don't need
the generic "lib/" subdir there.

Although an optional platform-specific subdir in ~ might be useful, if
you run pcb on multiple architectures with the same $HOME.

That would be ~/.pcb/hids/i686-pc-linux-gnu/ on my machine.