Add MacPorts as an option to install, do not require Homebrew

[barracuda156]

@vmagnin I have added the port for this: https://ports.macports.org/port/gtk-fortran

And independently, perhaps it is better not to require Homebrew (or any package manager), since everything can be built from source without package managers, if desired. I refer to "You need the Homebrew package manager" here: https://github.com/vmagnin/gtk-fortran/wiki/Installation