trying to use this in a iOS 12.1 environment - getting an issue when I try and compile.

getting ->
 Undefined symbols for architecture arm64:
  "_png_init_filter_functions_neon", referenced from:
      _png_init_filter_functions in pngrutil.o
ld: symbol(s) not found for architecture arm64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

seems to defined in pngpriv.h. -> line 140

Any suggestions are welcomed.
