Unable to install unbundled on macOS

Hi,

I am a maintainer of Nixpkgs & trying to get Qt packaged better. Because
Nixpkgs runs on both macOS & Linux, we want to have some consistency
between the two. AFAICT Qt will only install in "framework" style on
macOS with no obvious way to disable this. This means that some basic
things like headers, libs, & pkgconfig files are missing (instead in the
.framework directory). If every app knew how to handle frameworks
correctly, this would be fine, but currently many apps either can't find
our frameworks or don't handle them at all. In fact, even some official
Qt projects like PySide don't find our frameworks. See these two logs,
one from a Linux build of PySide & another from the macOS build of
PySide.

Linux: https://hydra.nixos.org/build/73220919/nixlog/1
macOS: https://hydra.nixos.org/build/73059550/nixlog/1

I'm not sure if this is something that we are doing wrong in our
environment, or PySide not configured correctly to work on macOS. At
least it would be nicer for us if we could have both bundled & unbundled
installed so that errors like the above wouldn't happen.

Would it be possible to make this an option? Maybe something already
exists but I have not found it in the documentation. I've attached the
patch (originally from Homebrew) that we are currently using to get
PkgConfig & headers to install, but it is very hacky & specific to Qt
5.10.