doc: use perl via PATH when building manpage
doc: use perl via PATH when building manpage

Linuxisms do not work on *BSD

cwyang/h2o,devnexen/h2o,deweerdt/h2o,yannick/h2o,h2o/h2o,yannick/h2o,h2o/h2o,h2o/h2o,deweerdt/h2o,yannick/h2o,deweerdt/h2o,yannick/h2o,h2o/h2o,devnexen/h2o,cwyang/h2o,yannick/h2o,deweerdt/h2o,cwyang/h2o,cwyang/h2o,cwyang/h2o,cwyang/h2o,cwyang/h2o,yannick/h2o,h2o/h2o,h2o/h2o,h2o/h2o,h2o/h2o,devnexen/h2o,h2o/h2o,yannick/h2o,devnexen/h2o,deweerdt/h2o,devnexen/h2o,devnexen/h2o,devnexen/h2o,devnexen/h2o,deweerdt/h2o,yannick/h2o,deweerdt/h2o,cwyang/h2o,deweerdt/h2o,yannick/h2o,devnexen/h2o,deweerdt/h2o