Making some tweaks that would make me happy as a vcpkg maintainer

[Honeybunch]

I am currently working on getting steam-audio available via the vcpkg package manager (see microsoft/vcpkg#40942 ). However, I have had to make a substantial patch. In doing so I whipped up this PR which would minimize the work for a downstream maintainer like myself in the future.

• Added an option to disable closed source dependencies (vcpkg ports are intended to be built from source)
• Added an option to export the cmake package
• Made CMAKE_OSX variables override-able (so you can build just an arm64 or x86_64 library)
• Made the include directories compatible with project export
• Only installing pdb/dbg info on shared library build (just being nice for vcpkg)

When the project is marked for export the minimal dependencies linked will be fetched via config mode so they can be tracked during the export process and the dependency chain will be properly exported. In this case the dependencies (mysofa, flatbuffers, pffft) will be found via Config mode which vcpkg or another package manager should provide via its environment.

Happy to make any changes you see fit to this. I'm not married to any of it. Also, if you don't want me to make steam-audio available on vcpkg at all let me know and I can just drop this 😄