Build libpng with Xcode

build-libpng-xcode.sh

# install command line developer tools
xcode-select --install

# update macports
sudo port selfupdate; sudo port upgrade outdated

# install cmake
sudo port install cmake

# download and extract libpng
curl -O ftp://ftp.simplesystems.org/pub/libpng/png/src/libpng16/libpng-1.6.12.tar.xz
tar xvf libpng-1.6.12.tar.xz
cd libpng-1.6.12

# make project files
cmake -G Xcode

# configure and make and install
./configure
make
sudo make install

# header files and binaries
ls /usr/local/include/libpng16/
ls /usr/local/lib/libpng*

# open in xcode
open libpng.xcodeproj/