These are the steps:
$ sudo mkdir /opt/cinnamon-dev
$ sudo chown $(id -nu).$(id -ng) /opt/cinnamon-dev
$ apt-get build-dep muffin cinnamon
$ cd
$ git clone git://github.com/linuxmint/muffin.git
$ cd muffin
$ autoreconf --force --install
$ ./configure --prefix=/opt/cinnamon-dev
$ make
$ make install
$ cd
$ git clone git://github.com/linuxmint/Cinnamon.git
$ cd Cinnamon
$ autoreconf --install --force
$ aclocal
$ intltoolize --force
$ env PKG_CONFIG_PATH=/opt/cinnamon-dev/lib/pkgconfig CFLAGS="-w" ./configure --prefix=/opt/cinnamon-dev
$ make
$ make install
# FAIL!
$ cd files/
$ mkdir -p opt/cinnamon-dev
$ mv usr opt/cinnamon-dev/
$ mv etc/ opt/cinnamon-dev/
$ cd ..
$ make install
# Now works!
The ugly "moves" after the fails are needed because I think some paths are hardcoded, and the process that copies the files from the "files/" directory ignores the "--prefix" configured.
