build libpng with libm
build libpng with libm

koreader/koreader-base,houqp/koreader-base,apletnev/koreader-base,frankyifei/koreader-base,Frenzie/koreader-base,NiLuJe/koreader-base,houqp/koreader-base,houqp/koreader-base,Frenzie/koreader-base,Hzj-jie/koreader-base,NiLuJe/koreader-base,Frenzie/koreader-base,Hzj-jie/koreader-base,frankyifei/koreader-base,koreader/koreader-base,koreader/koreader-base,frankyifei/koreader-base,frankyifei/koreader-base,NiLuJe/koreader-base,apletnev/koreader-base,apletnev/koreader-base,apletnev/koreader-base,NiLuJe/koreader-base,Frenzie/koreader-base,Hzj-jie/koreader-base,houqp/koreader-base,koreader/koreader-base,Hzj-jie/koreader-base