The default on MacOS is `md5`, while Nix only has `md5sum` available.
Therefore, make it possible to override the variable via the environment
so that flake.nix can set the correct binary name.
This allows us to test and use LLVM 17, now that it is available in
Homebrew.
Full support for LLVM 17 (including using it by default) will have to
wait until Espressif rebases their Xtensa fork of LLVM.
Previously all (except one!) usage of goenv.Version manually added the
git sha1 hash, leading to duplicate code. I've moved this to do it all
in one place, to avoid this duplication.
This is a big change: apart from removing LLVM 14 it also removes typed
pointer support (which was only fully supported in LLVM up to version
14). This removes about 200 lines of code, but more importantly removes
a ton of special cases for LLVM 14.
Prior to this commit, the build instructions were encoded in Makefiles
that contained GNU extensions that are unique to GNU Make and
incompatible with older implementations.
Thie commit renames all Makefiles to GNUmakefile which clearly denotes
that the file contains GNU extensions.
GNU Make actually first looks for a GNUmakefile, then makefile, THEN
Makefile so in addition to simply being more correct and portable, it
saves a few unnecessary failed attempts to open the correct build
file.
