Missing instructions on Linux (Unix) build dependencies

[csqn2]

Hi,
the good news is that the Makefile.unix still works on Debian 12 Bookworm (testing) in 2023 with the build-essential package installed.

Bad news is that it failed at first and I had to install the following libraries for the build to finish successfully (I hope I listed them all):
sudo apt-get install libx11-dev libxpm-dev libfltk1.3-dev libxft-dev libxcursor-dev libxinerama-dev libxfixes-dev libfontconfig-dev libxext-dev libpng-dev liblz-dev libjpeg-dev

This was not discernible at all from the English readme and I had to figure these out one by one. Hope this helps someone or that the readme gets updated.

P.S. That is a lot of X dependencies for a supposedly command-line application.