CC="gcc $CFLAGS" : This makes libpng use CFLAGS when compiling. ZLIBINC=/usr/include ZLIBLIB=/usr/lib : This forces libpng to look for the zlib includes and libraries where we have them installed. -f scripts/makefile.linux : This points make at the the Linux version of the makefile as libpng doesn't use a autoconf routine instead coming with various makefiles for different platforms.