make PIRC load-on-demand component

[plobsing]

Originally reported in parrot TT #42 (http://trac.parrot.org/parrot/ticket/42)

As suggested by pmichaud (I think PDS2008), it would be nice to have the PIR compiler an optional component, which is loaded only when needed.

Making this happen with IMCC is not worth the effort, but for PIRC this can be done.

The idea is that Parrot will load the PIR compiler as a dynamic library (.so, .dll), only when PIR input is fed into it; if it's PBC, the no PIR compiler is needed. This will also help to lower the memory footprint when running on smaller devices.

Alternatively, this could be a build (configure) option.