INSTALL.md updates for macOS

I plan on resolving this issue with a PR soon, but am opening this to summarize what I've found so far and to ask a few questions.

I'm not sure how much detail I should add to the macOS section of INSTALL.md: should I explicitly write out the brew install commands with the dependencies if said dependencies vary based on the version of macOS? should I include instructions to configure PATH in zsh to shadow apple clang (and its aliases) with another compiler?

the command realpath doesn't exist on macOS Monterey or earlier (only available in /usr/bin from Ventura onwards)

brew install coreutils fixes this (/opt/homebrew/bin needs to be on PATH)

subproject knarc does not build on apple clang 14.0.0 (Monterey) because it is not c++20 compliant (not sure if the version of apple clang bundled with xcode cmdline tools for Ventura/Sonoma is c++20 compliant enough to build knarc)

any c++20 compliant compiler should work

apple clang is very annoying because it has unremovable aliases gcc and g++—meaning any other compiler masquerading as gcc (whether it's homebrew gcc or homebrew clang) must be earlier in path than /usr/bin, since meson looks for gcc/g++ first and will find one if apple clang is installed

this isn't necessarily all that complicated to resolve with a symlink if you know what you're doing, but it's a lot of steps for an inexperienced user to follow

libpng can't build from source during configuration when it's not already already installed (for nitrogfx)

brew install pkg-config libpng fixes this

also there's some configurations I can't currently test (if no one else gets around to it, I can probably test on macOS 13 by the end of the summer because I was planning on updating soon anyway)

INSTALL.md

Mac OSX (<=10.14)

is this supposed to be for macOS Mojave and older? (if this works on those, i'll add a supplementary section for more recent versions instead of replacing this entirely)

macOS 13 Ventura (and higher?)

it may be the case that xcode command line tools (plus the already listed dependencies) is enough
realpath is apparently included in /usr/bin from here onwards (no coreutils)
bundled compiler is a newer version of apple clang that should be c++20 compliant
(needs verification) libpng may or may not build from source on Ventura when libpng and pkgconfig aren't already installed via brew

macOS 12 Monterey (and lower?)

xclt is still necessary (homebrew prompts you to get it on install)
it is technically possible to uninstall it after installing homebrew, but this solves nothing as /usr/bin contains wrapper binaries that prompt you to install xclt (which execute the real binaries in /Library/Developer/CommandLineTools) which includes gcc, g++, clang, etc. (which prompt xclt install or redirect to the actual clang executable) — so they still show up in PATH

homebrew gcc-14 as c++20 compliant compiler

gcc-14/g++-14 will need symlinks with the names gcc/g++ for the current build system to find them instead of apple clang

homebrew clang-18 as c++20 compliant compiler

clang-18 also needs symlinks with the names gcc/g++ for the current build system to find it instead of apple clang
llvm clang is masquerading as gcc (which may not be a big deal because apple clang already does that)

working configurations:

gcc in path is actually gcc-14 — pkg-config and libpng brewed
gcc in path is actually clang-18 — pkg-config and libpng brewed

failing configurations:

aliasing gcc to a compiler other than apple clang — meson ignores aliases
earliest gcc in path is actually gcc-14 — subproject libpng doesn't build
earliest gcc in path is actually clang-18 — subproject libpng doesn't build
gcc-14 without gcc symlink — meson picks apple clang
clang-18 without gcc symlink — meson picks apple clang

configurations left to test:

something about CC and CXX environment variables? — fairly certain meson doesn't actually check this (but I don't know how to use them in the first place)
cmake+libpng instead of pkgconfig+libpng

pret / pokeplatinum

INSTALL.md updates for macOS #246