How to build Xorg unintrusively. Works on Debian Sid with xdm. At the time of posting this, at least.

xorg_build-0.5.sh

#!/bin/bash
## Thank you for helping us help you help us all

SRCDIR=/linux1/builds
DSTDIR=/linux1/usr

## Define what's cloned

REPOS="\
    git://anongit.freedesktop.org/git/xorg/util/macros \
    git://anongit.freedesktop.org/git/xorg/proto/x11proto \
    git://anongit.freedesktop.org/git/xorg/lib/libXau \
    git://anongit.freedesktop.org/git/xorg/lib/libxshmfence \
    git://anongit.freedesktop.org/git/xorg/lib/libxtrans \
    git://anongit.freedesktop.org/xorg/xserver \
    git://anongit.freedesktop.org/git/xorg/driver/glamor \
    git://anongit.freedesktop.org/xorg/driver/xf86-video-ati \
    git://anongit.freedesktop.org/xorg/driver/xf86-input-evdev \
    git://anongit.freedesktop.org/pixman \
    git://anongit.freedesktop.org/xorg/proto/dri3proto \
    git://anongit.freedesktop.org/xorg/proto/presentproto \
    git://anongit.freedesktop.org/xorg/proto/glproto \
    git://anongit.freedesktop.org/xorg/proto/xextproto \
    git://anongit.freedesktop.org/xcb/pthread-stubs \
    git://anongit.freedesktop.org/xcb/proto \
    git://anongit.freedesktop.org/xcb/libxcb \
"

## Set up the environment
export DRI_DRIVERS="radeon,r200"
export GALLIUM_DRIVERS="r300,r600,swrast"

export PKG_CONFIG_PATH=$DSTDIR/lib/pkgconfig:$DSTDIR/share/pkgconfig:${PKG_CONFIG_PATH}
export LD_LIBRARY_PATH=$DSTDIR/lib:${LD_LIBRARY_PATH}
export LD_RUN_PATH=$DSTDIR/lib:${LD_RUN_PATH}
export LDFLAGS=-L$DSTDIR/lib CPPFLAGS=-I$DSTDIR/include
export ACLOCAL="/usr/bin/aclocal -I $DSTDIR/share/aclocal"

# Abort on errors
set -e

## Helpers

function _help() {
    echo "USAGE: $1 [--skip-deps] [--skip-source]"
    exit 0
}

function _straightforward_build() {
    cd $1
    ./autogen.sh --prefix=$DSTDIR
    sudo make install
    sudo /sbin/ldconfig
    cd ..
}

function _check_arg() {
    arg=$1
    args=${@:2}

    ## 0 is true
    for _arg in $args; do
        test $arg == $_arg && return 0
    done

    # the cake is a lie
    return 1
}

## Actual functionality

function install_deps() {
    sudo apt-get build-dep libdrm mesa xserver-xorg-video-ati xorg-server
    sudo apt-get install git llvm-3.4-dev libelf-dev linux-headers-$(uname -r) build-essential
    sudo apt-get install libevdev-dev
}

function get_source() {
    test -d $SRCDIR || mkdir -p $SRCDIR
    cd $SRCDIR

    for repo in $REPOS; do
        dirname=$(basename $repo)
        test -d $dirname && {
            cd $dirname
            git pull
            cd ..
        } || git clone $repo
    done
}

function build_deps() {
    cd $SRCDIR

    _straightforward_build macros
    _straightforward_build libXau
    _straightforward_build libxtrans
    _straightforward_build x11proto
    _straightforward_build dri3proto
    _straightforward_build presentproto
    _straightforward_build glproto
    _straightforward_build xextproto
    _straightforward_build pthread-stubs
    _straightforward_build proto
    _straightforward_build libxcb
    _straightforward_build libxshmfence
    # _straightforward_build drm
    _straightforward_build glamor
}

function build_mesa() {
    cd mesa
    ./autogen.sh --prefix=$DSTDIR \
        --with-dri-drivers=$DRI_DRIVERS --with-gallium-drivers=$GALLIUM_DRIVERS \
        --with-egl-platforms=x11,drm --enable-gbm --enable-shared-glapi \
        --enable-glx-tls.
    make
    sudo make install
    cd ..
}

function build_ati_evdev_pixman() {
    _straightforward_build xf86-input-evdev
    _straightforward_build xf86-video-ati
    _straightforward_build pixman
}

function build_xserver() {
    cd xserver
     ./autogen.sh --prefix=$DSTDIR --enable-xorg \
         --disable-dmx --disable-xvfb --disable-xnest --disable-xwin
    make
    sudo make install
    cd ..
}

function cake_is_served() {
    sudo chown root $DSTDIR/bin/Xorg
    sudo chmod u+s $DSTDIR/bin/Xorg
    sudo ldconfig
    sudo ln -sf /usr/bin/xkbcomp $DSTDIR/bin/xkbcomp 
    sudo ln -sf /usr/share/X11/xkb/rules $DSTDIR/share/X11/xkb/rules
}

function grief_counselling() {
    cat <<EOF
Add to /etc/environment:

    LIBGL_DRIVERS_PATH=$DSTDIR/lib/dri/

Add to /etc/X11/xorg.conf:

    Section "Files"
    ModulePath "$DSTDIR/lib/xorg/modules,/usr/lib/xorg/modules"
    EndSection

    Section "Module"
    Load "dri2"
    Load "glamoregl"
    EndSection

… and in the Device section for your video card add Option "AccelMethod" "glamor" and make sure Driver is set to "radeon".

Create /etc/ld.so.conf.d/0-xorg-git.conf:

    $DSTDIR/lib

Have this in /etc/X11/xdm/Xservers

    :0 local /opt/xorg/bin/X :0 vt7 -nolisten tcp

EOF
}

function main() {
    test ! -z $1 && {
        test $1 == "--help" || test $1 == "-h" && _help
    }

    # Conditionally set up the env
    # _check_arg "--skip-deps" $@ || install_deps
    _check_arg "--skip-source" $@ || get_source

    # Always build everything, though
    build_deps
    # build_mesa
    # build_ati_evdev_pixman
    # build_xserver
    # cake_is_served
    # grief_counselling
}

main $@