'make' fails with the following:

[...]

dot -Tpng -Tpng -Gratio=.9 -Gnodesep=.005 -Granksep=.00005 -Nfontsize=9 -Nheight=.1 -Nwidth=.1 < "doc/images/bootstrap-graph.dot" > "doc/images/bootstrap-graph.png.tmp"
/bin/bash: dot: command not found
make[2]: *** [doc/images/bootstrap-graph.png] Error 127