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

[vmagnin]

Thanks a lot @barracuda156 for that contribution!
Do you envision also a gtk4 branch version?

Concerning the installation instructions, I am not familiar with macOS. If you can help reformulating that whole section for the benefit of mac users, your help would be welcome.