Add commands to build dynamiclib.

The library files produced by the initial recipe did not work with mkmf,
so it was not possible to build a gem to wrap the libstree library.

The post at https://rt.cpan.org/Public/Bug/Display.html?id=80233
describes a similar problem with the solution of using the macports
version of libstree instead of building from source.

Examining the Portfile, it showed the following changeset:
https://trac.macports.org/changeset/15887
introduced a script to build a .dylib which mkmf can link against. The
effect of this script was duplicated in the homebrew formula and mkmf is
now able to find symbols in the library.