Requires very recent toolchain, because ATmega32U2 is relatively new.

Building:

make
make upload prog

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

Making the toolchain:

# patches according to
# http://www.avrfreaks.net/index.php?name=PNphpBB2&file=viewtopic&p=789527

# some gcc prerequisites

apt-get remove avr-libc gcc-avr binutils-avr
apt-get install libmpfr-dev libmpc-dev

# binutils

wget http://ftp.gnu.org/gnu/binutils/binutils-2.21.tar.bz2
tar xfj binutils-2.21.tar.bz2 
cd binutils-2.21
./configure --target=avr --disable-nls
make
make install

# gcc

wget http://ftpmirror.gnu.org/gcc/gcc-4.5.2/gcc-4.5.2.tar.bz2
wget -O gcc_452_avr.patch http://gcc.gnu.org/bugzilla/attachment.cgi?id=23050
tar xfj gcc-4.5.2.tar.bz2
cd gcc-4.5.2
patch -p1 -s <../gcc_452_avr.patch
mkdir obj-avr
cd obj-avr
../configure --target=avr --enable-languages=c \
    --disable-nls --disable-libssp --with-dwarf2
make
make install

wget http://download.savannah.gnu.org/releases/avr-libc/avr-libc-1.7.1.tar.bz2
tar xfj avr-libc-1.7.1.tar.bz2 
cd avr-libc-1.7.1
./bootstrap	# the automake at the end takes a while
./configure --build=`./config.guess` --host=avr
make
make install