if(APPLE)
  find_package(PNG REQUIRED)
  find_library(CORE_FOUNDATION CoreFoundation REQUIRED)
elseif(LINUX OR ANDROID)
  if(LINUX_PKG)
    find_library(PNG_LIBRARY libpng.so)
  else(LINUX_PKG)
    find_library(PNG_LIBRARY libpng.so PATHS ${CMAKE_EXTERNAL_LIB} NO_DEFAULT_PATH NO_CMAKE_FIND_ROOT_PATH)
  endif(LINUX_PKG)
elseif(WINDOWS)
  find_debug_and_optimized_library(PNG_LIBRARY "libpng/debug" "libpng" "libpng/release" "libpng")
endif(APPLE)