Update built on OS X to 1.6.19.

This was SVN commit r17291.

build/premake/extern_libs4.lua

@@ -456,7 +456,7 @@ extern_lib_defs = {
 				unix_names = { "png" },
 				-- Otherwise ld will sometimes pull in ancient 1.2 from the SDK, which breaks the build :/
 				-- TODO: Figure out why that happens
-				osx_names = { "png15" },
+				osx_names = { "png16" },
 			})
 		end,
 	},

libraries/osx/build-osx-libs.sh

@@ -32,7 +32,7 @@ JPEG_VERSION="jpegsrc.v8d"
 JPEG_DIR="jpeg-8d" # Must match directory name inside source tarball
 # * libpng was included as part of X11 but that's removed from Mountain Lion
 #   (also the Snow Leopard version was ancient 1.2)
-PNG_VERSION="libpng-1.5.13"
+PNG_VERSION="libpng-1.6.19"
 OGG_VERSION="libogg-1.3.0"
 VORBIS_VERSION="libvorbis-1.3.3"
 # gloox is necessary for multiplayer lobby