Add `Portfile` to support MacPorts

[marcransome]

Tasks to complete:

• Create basic Portfile definition
• Add unit tests and update cmake configuration; see Restructure codebase and add unit tests #46
• Add test options test.run, test.cmd and test.target to Portfile accordingly; see portfile(7) for more information
• Update Portfile checksums to represent the latest release tarball (openssl dgst -rmd160 <file> and openssl dgst -sha256 <file>)

[marcransome]

An initial Portfile definition has been added to a forked ports tree to add support for MacPorts.