The script:

------------------------------

#!/bin/sh

# Set initial variables
CWD=`pwd`
if [ "$TMP" == "" ]; then
	TMP=/tmp
fi
VERSION=0.7.2
PKG_VERSION=$VERSION
ARCH=i686
BUILD=${BUILD:-1_bbl}
APP=rtorrent
PKG=$TMP/package-$APP
if [ "$ARCH" == "i686" ]; then
	SLKFLAGS="-O2 -march=i686 -mtune=i686"
fi

# Clean up old files
rm -rf $PKG
mkdir -p $TMP $PKG
rm -rf $TMP/$APP-$VERSION

# Go to /tmp, unpack sources, copy them
cd $TMP || exit 1
tar -xf $CWD/$APP-$VERSION.tar.gz
cd $APP-$VERSION || exit 1
chown -R root:root .
chmod -R u+w,go+r,a-s .
CFLAGS="$SLKFLAGS" \
./configure \
	--prefix=/usr \
	--sysconfdir=/etc \
	--localstatedir=/var \
make || exit
make install DESTDIR=$PKG || exit
mkdir -p $PKG/usr/doc/$APP-$VERSION
cp -a BUGS Changes FAQ INSTALL LICENCE MANIFEST README TODO docs/ $PKG/usr/doc/$APP-$VERSION
find $PKG/usr/doc/$APP-$VERSION -type f -exec chmod 644 {} \;
cat $CWD/$APP.SlackBuild > $PKG/usr/doc/$APP-$VERSION/$APP.SlackBuild
( cd $PKG
	find . | xargs file | grep "executable" | grep "ELF" | cut -f 1 -d : | xargs strip --strip-unneeded 2> /dev/null
	find . | xargs file | grep "shared object" | grep "ELF" | cut -f 1 -d : | xargs strip --strip-unneeded 2> /dev/null
)
if [ -d $PKG/usr/man ]; then
	( cd $PKG/usr/man
		find . -type f -exec gzip -9 {} \;
		for i in `find . -type l` ; do ln -s $( readlink $i ).gz $i.gz ; rm $i ; done	
	)
fi
if [ -d $PKG/usr/info ]; then
	gzip -9 $PKG/usr/info/*.info
	rm -f $PKG/usr/info/dir
fi

# Prepare install directory
mkdir -p $PKG/install
cat $CWD/slack-desc > $PKG/install/slack-desc

# Build the package
cd $PKG
/sbin/makepkg -l y -c n $TMP/$APP-$PKG_VERSION-$ARCH-$BUILD.tgz

------------------------------

The error:

checking for C++ compiler default output file name... a.out
checking whether the C++ compiler works... yes
checking whether we are cross compiling... no
checking for suffix of executables... 
checking for suffix of object files... o
checking whether we are using the GNU C++ compiler... yes
checking whether g++ accepts -g... yes
checking for style of include used by make... GNU
checking dependency style of g++... gcc3
checking build system type... Invalid configuration `make': machine `make' not recognized
configure: error: /bin/sh ./config.sub make failed