You probably might have noticed, the citadel team provides debian/ubuntu binary packages.
Anyway, Easy Install requires a working build environment. This is installed by default on nearly all modern Linux distributions. The notable exception to this is Ubuntu, which does not ship with a working build environment. To install a build environment use the following commands (as root):

apt-get update
apt-get install build-essential curl g++ gettext shared-mime-info

If you want a Citadel installation which supports SSL encryption (recommended) you will need to install the development packages for OpenSSL. On Debian and Ubuntu, this can be installed with the following commands (as root):

apt-get update
apt-get install libssl-dev

If you want to use for example GNOME File-Icons in Webcit, install

apt-get install gnome-icon-theme

and after installation do

cd /usr/local/webcit/static
ln -s /usr/share/icons/gnome/24x24/mimetypes icons

You can also use every other Freedesktop.org compliant Icon theme.