How to set QTDIR environment variable

[Nigel Henry <cave.dnb2m97pp@aliceadsl.fr>]

I'm trying to build pdfedit, which isn't available for Etch. I've already 
dealt with some deps (libboost-dev, libboost-iostreams-dev, 
libboost-iostreams1.33.1, and libt1-dev), but ./configure is complaining 
about the gui about the QT gui as below.

checking whether we want to build PDFedit gui... yes
configure error: QTDIR environment variable must be set

I'm sure I've dealt with these QT problems before, but could someone refresh 
my memory please as to how to set the QTDIR environment variable.

Many thanks for any help.

Nigel.