You need to install -dev package for zlib1g - it is named zlib1g-dev:

sudo apt-get install zlib1g-dev

and it will install zlib.pc file for pkg-config along with zlib.h header.

sudo apt-get install zlib1g-dev

I met the same issue when built other libraries. Just install zlib1g-dev solved my problem.

This seems to be a bug in CMake: https://gitlab.kitware.com/cmake/cmake/-/issues/20127.

Install zlib with development support by using

sudo apt-get install zlib1g-dev

In case you don't want or need to use the full zlib, it is fairly easy to write wrapper routines which map the zlib functions 1:1 to ordinary file functions which don't support compression and decompression.

//
//  dummy zlib.h
//

#pragma once
#include <stdio.h>

typedef FILE *gzFile;

int gzclose(gzFile file);
gzFile gzdopen(int fd, const char *mode);
gzFile gzopen(const char *path, const char *mode);
int gzread(gzFile file, void *buf, unsigned int len);


//
//  zlibDummy.cpp
//

#include <zlib.h>

int gzclose(gzFile file)
{
    return fclose(file);
}

gzFile gzdopen(int fd, const char *mode)
{
    return _fdopen(fd, mode);
}

gzFile gzopen(const char *path, const char *mode)
{
    return fopen(path, mode);
}

int gzread(gzFile file, void *buf, unsigned int len)
{
    return fread(buf, 1, len, file);
}

You appear to be compiling something from source. You may find this is unneccessary, as the thing you need may be already built and packaged in binary form.

sudo apt install zlib1g

If, however, you're building some other piece of software that requires zlib, then:

sudo apt install zlib1g-dev

Which will install the development headers required by your other software.

To me this looks like CMake is not complaining about missing include (ZLIB_INCLUDE_DIR) but the zlib library itself...? Would installing the lib itself (zlib1g?) help?

@gamaliel
Qt4 & Ubuntu 12 are quite old now. I don't know exactly why your installation shows this issue if others did not in the past. If you do not get an exact answer, however, you might want to:

• run ldd libQtGui.so to discover where it is looking for the missing zlib.so library.
• locate zlib.so to find where yours is installed.
• Often doing an ln -s from where it actually is to where Qt is expecting it to be will suffice for you.
• Or, I think environment variable LD_LIBRARY_PATH is read at start up to determine where to look for shared libraries, you can alter that for your Qt app.

There's also a way of building a zlib included with Qt4 itself (though see https://stackoverflow.com/questions/10692988/static-qt-4-8-1-build-on-ubuntu-12-04), though I presume you're not wanting to do that.

You should install the development support files for zlib, try:

sudo apt-get install libz-dev

^{Other package names: zlib1g-dev.}

If you've already zlib library, make sure you're compiling your code sources with -lz. See: missing zlib.h in ubuntu.