prepare for the time when all packages will export their docdir
prepare for the time when all packages will export their docdir

jmirabel/hpp-doc,laas/hpp-doc,humanoid-path-planner/hpp-doc,humanoid-path-planner/hpp-doc,jmirabel/hpp-doc,florent-lamiraux/hpp-doc,florent-lamiraux/hpp-doc,laas/hpp-doc