In order to build (Re)nouveau, you must have libSDL 1.2 installed. The easiest way to do this is using apt-get:

apt-get install libsdl1.2-dev

Installing the following will help you build nouveau.

apt-get install xorg-dev libdrm-dev git git-core libtool mesa-common-dev automake autoconf 

The Debian nvidia-glx package (at least the one in Etch) stores the original Mesa libglx as /usr/lib/nvidia/, which you may want to symlink as /usr/lib/xorg/modules/extensions/ to avoid the undefined symbol error.

Running git commands (such as git clone and git checkout) may yield the following message:

If you are looking for git, Linus Torvald's content tracker, install
the cogito and git-core packages and see README.Debian and git(7).

This transition script will be removed in the debian stable
release after etch.

If you wish to complete the transition early, install git-core
and use (as root):
 update-alternatives --config git

Press RETURN to run gitfm

If this is the case, run update-alternatives --config git as root, and select the /usr/bin/git-scm alternative (option 1 in the following example):

%) update-alternatives --config git

There are 2 alternatives which provide `git'.

  Selection    Alternative
      1        /usr/bin/git-scm
*+    2        /usr/bin/git.transition

Press enter to keep the default[*], or type selection number: