Add a new build() function to StaticDigraph (#68)

This function builds the digraph from an arc list that
contains pairs of integer indices from the range [0..n-1].
It is useful in the cases when you would like to build a
StaticDigraph from scratch, i.e. you do not want to build
another digraph that can be copied using the other build()
function.

 SET(LEMON_VERSION "@PACKAGE_VERSION@" CACHE STRING "LEMON version string.")