Best way to support multi-arch builds on osx for ports that don't support it

[omartijn]

PR #18156 adds support for setting multiple architectures in VCPKG_OSX_ARCHITECTURES. This works great for ports that use CMake themselves, but does not work for ports that don't.

The problem is mostly with ports using Autotools, since they tend to revolve around a config.h file which controls what is included and which options are enabled instead of looking at the preprocessor definitions. This means that it'll try to use the config for a specific platform also when compiling for the other.

One example of such a port is ffmpeg. If you add multiple -arch options to the configure (e.g. -arch x86_64 -arch arm64) the build fails with some specific byte-swapping code that works only for a single arch. ffmpeg is clearly not interested in fixing this, looking at messages on their mailing list this is apparently how they like it.

This means that we will need to do two separate builds and then use lipo to join everything into a single, universal binary.

It's likely that there are many other ports which have this same issue, so it could be beneficial to create something that can be used between different ports.