OS X build instructions need a little refresher

When building uTox on OS X, we need to specify lodepng within the png directory, or we get the following:

```
 cc -o uTox.o *.c -L/usr/local/lib -I/usr/local/include -I/opt/X11/include -L/opt/X11/lib -lX11 -lXrender -lXext -ltoxcore -ltoxav -ltoxdns -framework OpenAL -pthread -lresolv -ldl -lm -lfontconfig -lfreetype -lvpx -I/opt/X11/include/freetype2
Undefined symbols for architecture x86_64:
  "_lodepng_decode32", referenced from:
      _png_to_image in main-4ef908.o
  "_lodepng_encode_memory", referenced from:
      _doevent in main-4ef908.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
```

Also, I added in an extra line for those using homebrew to provide libtoxcore.