message: fix libpng neon detection for m1
  libpng neon detection is broken and must be explicitely forced at `cmake ..` time with a flag
diff: @@ -16,7 +16,9 @@ FetchContent_Declare(libpng
  INSTALL_DIR "${CMAKE_CURRENT_BINARY_DIR}/inst"
  LOG_DIR "${CMAKE_CURRENT_BINARY_DIR}/log"
  )
  +if(CMAKE_APPLE_SILICON_PROCESSOR STREQUAL "arm64")
  set(PNG_ARM_NEON "off" CACHE STRING "" FORCE)
  +endif()
  set(PNG_SHARED OFF CACHE BOOL "" FORCE)
  FetchContent_MakeAvailable(libpng)
  