I used this method to compile the FFmpeg binary for inclusion in my Serviio package, because I don’t have a NAS with an Intel CPU. Several people who tried to help were getting strange errors while trying to compile on their NAS units.

To begin, install 64bit Ubuntu Linux (I used a VM). As a test I first successfully cross-compiled ARM binaries using the same method but using the ARM Synology toolchain, since I was able to test the results. Once I got it working I did the following:

#-----set up Synology toolchain
cd ~/Downloads
wget http://sourceforge.net/projects/dsgpl/files/DSM%203.1%20Tool%20Chains/Intel%20x86%20Linux%202.6.32/gcc420_glibc236_pineview.tgz
tar xvfz gcc420_glibc236_pineview.tgz
sudo mv i686-linux-gnu /usr/local
sudo mv x86_64-linux-gnu /usr/local
#-----I think the Synology toolchain is anticipating being run on i686-linux-gnu, so we need 32bit libraries to even run the binaries
#-----http://maketecheasier.com/run-32-bit-apps-in-64-bit-linux/2009/08/10
sudo apt-get install ia32-libs
export PATH="/usr/local/x86_64-linux-gnu/bin:${PATH}"

#-----libz static lib
wget http://zlib.net/zlib-1.2.5.tar.gz
tar xvfz zlib-1.2.5.tar.gz
cd zlib-1.2.5
#-----http://www.crosscompile.org/static/pages/ZLib.html
#-----http://tinc-vpn.org/examples/cross-compiling-64-bit-windows-binary/
./configure --prefix=/usr/local/x86_64-linux-gnu --static
sed -i 's/^CC=gcc/CC=x86_64-linux-gnu-gcc/g' Makefile
sed -i 's/^LDSHARED=gcc/LDSHARED=x86_64-linux-gnu-gcc/g' Makefile
sed -i 's/^CPP=gcc -E/CPP=x86_64-linux-gnu-gcc -E/g' Makefile
sed -i 's/^AR=ar rc/AR=x86_64-linux-gnu-ar rc/g' Makefile
sed -i 's/^RANLIB=ranlib/RANLIB=x86_64-linux-gnu-ranlib/g' Makefile
make libz.a
cp libz.a /usr/local/x86_64-linux-gnu/lib
cp *.h /usr/local/x86_64-linux-gnu/include/
cd ..

#-----libbz2 static lib
wget http://bzip.org/1.0.6/bzip2-1.0.6.tar.gz
tar xvfz bzip2-1.0.6.tar.gz
cd bzip2-1.0.6
sed -i 's/^CC=gcc/CC=x86_64-linux-gnu-gcc/g' Makefile
sed -i 's/^AR=ar rc/AR=x86_64-linux-gnu-ar rc/g' Makefile
sed -i 's/^RANLIB=ranlib/RANLIB=x86_64-linux-gnu-ranlib/g' Makefile
make
make install PREFIX=/usr/local/x86_64-linux-gnu
cd ..

#-----libmp3lame static lib
wget http://sourceforge.net/projects/lame/files/lame/3.98.4/lame-3.98.4.tar.gz
tar xvfz lame-3.98.4.tar.gz
cd lame-3.98.4
./configure --prefix=/usr/local/x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu --build=x86_64-linux-gnu --enable-static --disable-shared --disable-decoder --enable-nasm
make
make install

#-----libssl & libcrypto static libs
ipkg install openssl-dev
wget http://www.openssl.org/source/openssl-0.9.8p.tar.gz
tar xvfz openssl-0.9.8p.tar.gz
cd openssl-0.9.8p
./config --prefix=/usr/local/x86_64-linux-gnu no-shared
sed -i 's/^CC= gcc/CC= x86_64-linux-gnu-gcc/g' Makefile
sed -i 's/^AR= ar /AR= x86_64-linux-gnu-ar /g' Makefile
sed -i 's/^ARD=ar /ARD=x86_64-linux-gnu-ar /g' Makefile
sed -i 's/^RANLIB= \/usr\/bin\/ranlib/RANLIB=x86_64-linux-gnu-ranlib/g' Makefile
sed -i 's/^MAKEDEPPROG= gcc/MAKEDEPPROG= x86_64-linux-gnu-gcc /g' Makefile
make
make install
cd ..

#-----librtmp 2.4 static lib
wget http://download.serviio.org/opensource/rtmpdump-c58cfb3e9208c6e6bc1aa18f1b1d650d799084e5.tar.gz
tar xvfz rtmpdump-c58cfb3e9208c6e6bc1aa18f1b1d650d799084e5.tar.gz
cd rtmpdump
#-----move all static libs to a separate folder to force compiler to use them
#-----http://www.gossamer-threads.com/lists/apache/dev/265052
mkdir /tmp/lib
cp /usr/local/x86_64-linux-gnu/lib/*.a /tmp/lib
#-----fix Makefile (won't compile without libdl linked)
#-----http://forum.luahub.com/index.php?topic=2390.0
sed -i.bak -e '/^LIB_OPENSSL\=/s/lcrypto/lcrypto \-ldl/' Makefile
make CROSS_COMPILE=x86_64-linux-gnu- SYS=posix prefix=/usr/local/x86_64-linux-gnu INC=-I/usr/local/x86_64-linux-gnu XLDFLAGS=-L/tmp/lib SHARED=
make install prefix=/usr/local/x86_64-linux-gnu SHARED=
cd ..
#-----gather all static libs again ready for FFmpeg compile
cp /usr/local/x86_64-linux-gnu/lib/*.a /tmp/lib
#-----remove unsupported URL line from pkgconfig/librtmp.pc
#-----pkg-config --exists --print-errors librtmp
sed -i -e '/^URL/d' /usr/local/x86_64-linux-gnu/lib/pkgconfig/librtmp.pc
export PKG_CONFIG_PATH="${PKG_CONFIG_PATH}:/usr/local/x86_64-linux-gnu/lib/pkgconfig"

#-----FFmpeg 0.8.2 (slightly newer than the one at Serviio.org)
wget http://dl.dropbox.com/u/1188556/ffmpeg-HEAD-05a2673.tar.gz
tar xvfz ffmpeg-HEAD-05a2673.tar.gz
cd ffmpeg-HEAD-05a2673
#-----Intel CPU-specific options
#-----pkg-config needs to be defined because with cross-prefix it assumes x86_64-linux-gnu-pkg-config which doesn't exist, and then librtmp won't be detected
sudo apt-get install yasm
./configure --arch=x86_64 --enable-ssse3 --cross-prefix=x86_64-linux-gnu- --target-os=linux --prefix=/usr/local/x86_64-linux-gnu
--extra-cflags='-I/usr/local/x86_64-linux-gnu/include' --extra-ldflags='-L/tmp/lib' --enable-static --disable-shared --disable-ffplay
--disable-ffserver --enable-pthreads --enable-libmp3lame --enable-librtmp --pkg-config=pkg-config --extra-version=Serviio
make
make install