How do I install X headers?

[Mikkel Mastrup Jensen <mikkelmastrup@hotmail.com>]

Hi, I'm currently running Debian 3.1, kernel 2.6.16.16, NVIDIA drivers 
and Gnome as window manager. I installed X.org from backports as the 
first thing on my new system by typing "apt-get -t sarge-backports 
install x-window-server". Gnome (and X in general) runs fine, but when 
trying to install e17 (Enlightenment) from cvs I get this error:

> checking for X11/X.h... no
> configure: error: Cannot find X headers and libraries.

I've been told I need to install X headers to fix it, but the list of 
packages on http://backports.org/debian/pool/main/x/xorg-x11/ is just so 
large, I'm not sure which one (or more) I need to apt-get...

Could you help me by telling me which of the packages on the list I need 
to install?

Thanks, Mikkel