devkitARM install instructions are out of date

[WinterMute]

Things have moved around a little and installing been made easier since you wrote up https://github.com/pret/pokeruby/blob/master/INSTALL.md

Largely it's just a matter of removing the instructions to set environment variables (fwiw all platforms are now using /opt/devkitpro which allows us to package portlibs easily)

We also moved from the old msys unix tools on windows to msys2 primarily because of pacman but also because they're much better. msys2 also allows installing of gcc & git there - it may also allow building of agbcc although I haven't checked yet. I'm slightly concerned about your use of cygwin here since 1. it's not generally recommended for both msys2 & cygwin to be in use and 2. It seems to be difficult to uninstall it once it's on the system. WSL is obviously another option if it's available.

I can do a PR with changes later if you're amenable?

[PikalaxALT]

@WinterMute Can you review the current state of INSTALL.md? Is it more up-to-date?

[WinterMute]

Sorry to take so long looking at this, too much to do as usual.

Personally I'd prefer that the recommended approach on windows was using the installer & pre-configured msys2 rather than implying WSL is the preferred option as is done with the pre-requisites section. Currently the installer is 64bit only but 32bit packages are available if the devkitPro repos are added to a 32bit msys2 install (one of the many things on the TODO lists to sort out).

For macOS I would stay far, far away from recommending Homebrew, that's using a 10 ton wrecking ball to crack a nut. The only thing I had to do to get this to work there was build libpng and everything else worked OOB for me after installing Xcode Command Line Tools & the gba-dev group. I'd honestly be more inclined to drop libpng in the source tree rather than mess about with all the various ways to deal with building or installing it.