You don't need to modify that file; it's [Makefile.swig.gen] generated.
Are you sure about this? When I replace every occurrence of "howto" in Makefile.swig.gen with my own package name, it seems to work. However, when I leave this file as it is in gr-how-to-write-a-block, or remove it entirely, the build process fails. (At make or in bootstrap, respectively.)
After looking closely at that file, it looks like it should be generated, but my experiences say otherwise.