build: defer $MAKE choice to cmake for BSD compatibility
build: defer $MAKE choice to cmake for BSD compatibility

The doc target uses gmake-specific functionality and therefore we need
to propagate the choice of make engine through the whole build chain

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