using alternate installation of plplot

[Alain]

Hi

I have build and installed /opt/plplot5.9.2 and would like to use it instead
of my default plplot (5.9.0 from debian lenny), but i dont find how 
to configure this alternate path/package in octave.

i'd like to keep both to compare the results, and check if bugs vanished or
not :-)

any hint would be appreciated.
Alain