For STL imports, Shapesmith will automatically detect whether the 'coarsen' executable exists on the system and simplify imported meshes.
For mesh simplification we user the GTS library: http://gts.sourceforge.net/.
'coarsen' is a GTS example and is not part of the standard GTS package. Thus it is easier to build this locally and symlink the 'coarsen' command.
$ sudo apt-get install libglib2.0-dev
$ brew install glib
$ wget http://downloads.sourceforge.net/project/gts/gts/0.7.6/gts-0.7.6.tar.gz
$ tar xzf gts-0.7.6.tar.gz
$ cd gts-0.7.6
$ ./configure
$ make
$ sudo make install
$ sudo ldconfig
$ sudo ln -s $PWD/examples/coarsen /usr/local/bin/coarsen
You can verify the installation by coarsening a mesh:
$ cd /tmp
$ wget http://gts.sourceforge.net/samples/x_wing.gts.gz
$ gunzip x_wing.gts.gz
$ cat x_wing.gts | gts2stl > x_wing.stl # Make an STL file from GTS file
$ stl2gts < x_wing.stl # Make a GTS file from an STL
$ coarsen < x_wing.gts # Should output a GTS file