Configure error: could not find the zlib library

I am trying to install lalsuite on ubuntu according to the instructions here:

https://pycbc.org/pycbc/latest/html/install_lalsuite.html

Everything went smoothly until I ran

./00boot
./configure --prefix=${VIRTUAL_ENV}/opt/lalsuite --enable-swig-python

And the error I get is:

configure: error: could not find the zlib library

I am doing all this inside a virtual environment using virtualenv. I'm fairly certain I have the zlib library as I installed it with:

$ sudo apt install zlib1g

After a thorough search for a solution I found someone with the same problem:

https://ubuntuforums.org/showthread.php?t=2217198&page=2&s=3d14badd56b8ebb236dfc0970ba19435

But I do not really understand their solution. My interpretation of their fix was to do the following:

First reinstall pkg-config:

sudo apt-get update
sudo apt-get install pkg-config

Then write the line suggested by the answer:

echo 'export PKG_CONFIG=/usr/bin/pkg-config' >> ~/.bashrc

(I am doing all this inside the virtualenv by the way).

Then restart everything and try the same thing again:

./00boot
./configure --prefix=${VIRTUAL_ENV}/opt/lalsuite --enable-swig-python

Sadly the same error appears.

Any help is appreciated.