How to compile libpng with a non-system zlib

Question

I'm having some trouble building libpng with a zlib newer than the system's zlib. My system has zlib 1.2.7 installed. But I want to build libpng against the latest zlib which I have built in a separate directory. So I'm doing this:

cd zlib-1.2.13
./configure
make
cd ../libpng-1.6.40
export CFLAGS=-I../zlib-1.2.13
export LDFLAGS=-L../zlib-1.2.13
./configure

But still, in the pnglibconf.h generated by configure, PNG_ZLIB_VERNUM is defined as 0x1270, i.e. zlib 1.2.7 - the system's zlib version. What do I need to do to tell libpng's configure to use zlib 1.2.13 instead? As you can see above, I've set CFLAGS and LDFLAGS to point to zlib-1.2.13 but it doesn't seem to work. It still uses the system's zlib.

Answer 1

From the documentation:

If you wish to test with a particular zlib that is not first in the standard library search path, put ZLIBLIB, ZLIBINC, CPPFLAGS, LDFLAGS, and LD_LIBRARY_PATH in your environment before running "make test" or "make distcheck":

ZLIBLIB=/path/to/lib export ZLIBLIB
ZLIBINC=/path/to/include export ZLIBINC
CPPFLAGS="-I$ZLIBINC" export CPPFLAGS
LDFLAGS="-L$ZLIBLIB" export LDFLAGS
LD_LIBRARY_PATH="$ZLIBLIB:$LD_LIBRARY_PATH" export LD_LIBRARY_PATH