libpng: another msys compilation adjustment
libpng: another msys compilation adjustment

emacstheviking/lambdanative,gale320/lambdanative,emacstheviking/lambdanative,VincentToups/lambdanative,VincentToups/lambdanative,youprofit/lambdanative,attila-lendvai/lambdanative,youprofit/lambdanative,emacstheviking/lambdanative,VincentToups/lambdanative,attila-lendvai/lambdanative,attila-lendvai/lambdanative,VincentToups/lambdanative,emacstheviking/lambdanative,VincentToups/lambdanative,gale320/lambdanative,youprofit/lambdanative,youprofit/lambdanative,gale320/lambdanative,gale320/lambdanative,attila-lendvai/lambdanative,attila-lendvai/lambdanative,emacstheviking/lambdanative,gale320/lambdanative,youprofit/lambdanative