Improve macOS packaging

[cgay]

Currently there are a few problems with building, installing, running a release on macOS...

1. Given we don't yet have native support for Apple silicon, is there a way to build for the x86_64 architecture on an M3 Mac? For now I'm using the Build Release workflow on GitHub to build the release, but this requires manually editing the config.jam file and re-tarring the release because on the GitHub machine xcrun --show-sdk-path returns a version-specific path, currently pointing to version 14.2 of Xcode. (I set it to /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk since my understanding is that this is more general since installing Xcode will also install the command-line tools. We'll see.)
2. Untarring the release and running the binaries on macOS Sonoma 14.3.1 results in a security pop-up: "dylan-environment cannot be opened because the developer cannot be verified." This is easy enough to work around, once for each distinct executable. I think we need to sign our binaries.
3. Ideally we should be making a real macOS application packaged in a dmg.

[cgay]

Re: the first point, I am currently using this to build on macOS, thanks to help from housel and brucem:

CC=/usr/bin/clang DYLANCOMPILER=/Users/cgay/dylan/opendylan-2024.2pre/bin/dylan-compiler ./configure --prefix=${DYLAN}/opendylan-dev --host=x86_64-darwin CFLAGS="-O2 -arch x86_64"