gutenprint: forcibly disable ijs-config

ijs-config (ghostscript) is looked up in /usr/bin, if you've got a host
that has ghostscript with development headers installed then the build
fails when trying to link to the host libijs.
Forcibly disable ghostscript support since we lack a package.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>