Explicitely disable docs build as it fails when it's attempted (FreeBSD 11+)

perl ./doc/interpreter/mk-qthelp.pl octave.html doc/interpreter/octave_interpreter && \
qcollectiongenerator -qt4 doc/interpreter/octave_interpreter.qhcp -o doc/interpreter/octave_interpreter.qhc >/dev/null && \
rm -f doc/interpreter/octave_interpreter.qhcp doc/interpreter/octave_interpreter.qhp
qcollectiongenerator: could not find a Qt installation of '4'

Reported by: pkg-fallout
With hat: portmgr