I ran this from a modified slackbuild script:

#!/bin/sh
 
# Slackware build script for ristretto 
 
# (C) 2008 Michael Wagner <lapinours@web.de> 
# All rights reserved.
#
# Redistribution and use of this script, with or without modification, is
# permitted provided that the following conditions are met:
#
# 1. Redistributions of this script must retain the above copyright
#    notice, this list of conditions and the following disclaimer.
#
# THIS SOFTWARE IS PROVIDED BY THE AUTHOR ''AS IS'' AND ANY EXPRESS OR IMPLIED
# WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
# MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO
# EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
# PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS;
# OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
# WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
# OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
# ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
PRGNAM=ristretto
VERSION=0.0.91
BUILD=${BUILD:-1}
TAG=${TAG:-_SBo}
 
# Automatically determine the architecture we're building on:
if [ -z "$ARCH" ]; then
  case "$( uname -m )" in
    i?86) ARCH=i486 ;;
    arm*) ARCH=arm ;;
    # Unless $ARCH is already set, use uname -m for all other archs:
       *) ARCH=$( uname -m ) ;;
  esac
fi
 
CWD=$(pwd)
TMP=${TMP:-/tmp/SBo}
PKG=$TMP/package-$PRGNAM
OUTPUT=${OUTPUT:-/tmp}
 
if [ "$ARCH" = "i486" ]; then
  SLKCFLAGS="-O2 -march=i486 -mtune=i686"
  LIBDIRSUFFIX=""
elif [ "$ARCH" = "i686" ]; then
  SLKCFLAGS="-O2 -march=i686"
  LIBDIRSUFFIX=""
elif [ "$ARCH" = "s390" ]; then
  SLKCFLAGS="-O2"
  LIBDIRSUFFIX=""
elif [ "$ARCH" = "x86_64" ]; then
  SLKCFLAGS="-O2 -fPIC"
  LIBDIRSUFFIX="64"
else
  SLKCFLAGS="-O2"
  LIBDIRSUFFIX=""
fi
 
set -e
 
rm -rf $PKG
mkdir -p $TMP $PKG $OUTPUT
cd $TMP 
rm -rf $PRGNAM-$VERSION
tar xvjf $CWD/$PRGNAM-$VERSION.tar.bz2 
cd $PRGNAM-$VERSION
chown -R root:root .
chmod -R u+w,go+r-w,a-s .
 
./autogen.sh
CFLAGS="$SLKCFLAGS" \
  ./configure \
  --prefix=/usr \
  --libdir=/usr/lib${LIBDIRSUFFIX} \
  --docdir=/usr/doc/$PRGNAM-$VERSION \
  --build=$ARCH-slackware-linux
 
make 
make install DESTDIR=$PKG
strip --strip-unneeded $PKG/usr/bin/ristretto
 
mkdir -p $PKG/usr/doc/$PRGNAM-$VERSION
cp -a \
  AUTHORS COPYING ChangeLog NEWS README TODO \
    $PKG/usr/doc/$PRGNAM-$VERSION
cat $CWD/$PRGNAM.SlackBuild > $PKG/usr/doc/$PRGNAM-$VERSION/$PRGNAM.SlackBuild
chmod 644 $PKG/usr/doc/$PRGNAM-$VERSION/*
 
mkdir -p $PKG/install
cat $CWD/slack-desc > $PKG/install/slack-desc
cat $CWD/doinst.sh > $PKG/install/doinst.sh
 
cd $PKG
/sbin/makepkg -l y -c n $OUTPUT/$PRGNAM-$VERSION-$ARCH-$BUILD$TAG.${PKGTYPE:-tgz}

you will get the error:

main_window.c:39:28: fatal error: main_window_ui.h: No such file or directory
compilation terminated.
make[3]: *** [ristretto-main_window.o] Error 1
make[3]: *** Waiting for unfinished jobs....
mv -f .deps/ristretto-image.Tpo .deps/ristretto-image.Po
mv -f .deps/ristretto-wallpaper_manager.Tpo .deps/ristretto-wallpaper_manager.Po
mv -f .deps/ristretto-app_menu_item.Tpo .deps/ristretto-app_menu_item.Po
mv -f .deps/ristretto-gnome_wallpaper_manager.Tpo .deps/ristretto-gnome_wallpaper_manager.Po
mv -f .deps/ristretto-settings.Tpo .deps/ristretto-settings.Po
mv -f .deps/ristretto-xfce_wallpaper_manager.Tpo .deps/ristretto-xfce_wallpaper_manager.Po
mv -f .deps/ristretto-image_list.Tpo .deps/ristretto-image_list.Po
mv -f .deps/ristretto-thumbnail.Tpo .deps/ristretto-thumbnail.Po
mv -f .deps/ristretto-preferences_dialog.Tpo .deps/ristretto-preferences_dialog.Po
mv -f .deps/ristretto-thumbnail_bar.Tpo .deps/ristretto-thumbnail_bar.Po
mv -f .deps/ristretto-picture_viewer.Tpo .deps/ristretto-picture_viewer.Po
make[3]: Leaving directory `/tmp/SBo/ristretto-0.0.91/src'
make[2]: *** [all] Error 2
make[2]: Leaving directory `/tmp/SBo/ristretto-0.0.91/src'
make[1]: *** [all-recursive] Error 1
make[1]: Leaving directory `/tmp/SBo/ristretto-0.0.91'
make: *** [all] Error 2

Instead one must replace ./autogen.sh with ./configure  It appears its getting confused with the paths, autogen is not getting the same paths as ./configure