makefile2graph: New formula

Creates a graph of dependencies from GNU-Make;
Output is a graphiz-dot file or a Gexf-XML file.
Can this test be modified to do something more substantial than e.g. `--version` or `--help`? See `cmake.rb` for an example of an application formula with a good test and `tinyxml2.rb` for an example of a library formula with a good test. Thanks!