update binding.gyp to compile libpng
update binding.gyp to compile libpng

GrimDerp/lwip,sanemat/lwip,lpicanco/lwip,EyalAr/lwip,EyalAr/lwip,unbornchikken/lwip,cocos-creator/lwip,antoinepairet/lwip,oliversalzburg/lwip,lpicanco/lwip,mottx/lwip,MAMISHO/lwip,gimox/lwip,gimox/lwip,cocos-creator/lwip,oliversalzburg/lwip,fireball-x/lwip,domosapien/lwip,mottx/lwip,GrimDerp/lwip,oliversalzburg/lwip,unbornchikken/lwip,neophob/lwip,domosapien/lwip,GrimDerp/lwip,EyalAr/lwip,neophob/lwip,weareu/lwip,unbornchikken/lwip,michaelBenin/lwip,michaelBenin/lwip,weareu/lwip,sanemat/lwip,EyalAr/lwip,lpicanco/lwip,weareu/lwip,gimox/lwip,cocos-creator/lwip,gimox/lwip,antoinepairet/lwip,domosapien/lwip,MAMISHO/lwip,fireball-x/lwip,oliversalzburg/lwip,fireball-x/lwip,sanemat/lwip,antoinepairet/lwip,mottx/lwip,unbornchikken/lwip,cocos-creator/lwip,mottx/lwip,michaelBenin/lwip,sanemat/lwip,weareu/lwip,neophob/lwip,GrimDerp/lwip,michaelBenin/lwip,fireball-x/lwip,antoinepairet/lwip,neophob/lwip,lpicanco/lwip,MAMISHO/lwip,MAMISHO/lwip,domosapien/lwip