Rather than two lots of logic doing roughly the same thing, dependent on if
you're compiling on Linux or macOS, combine the .DS store generation into
macdeployqtplus.
This also removes the -fancy and -volname options.
Previously, we did not include the macOS SDK libc++ headers in our SDK
creation process and instead used whichever libc++ headers shipped with
the clang package we downloaded in depends.
This change adds a script (which works on both GNU/Linux and macOS) to
correctly generate the macOS SDK including the libc++ headers. This can
be thought of as a simplified rewrite of tpoechtrager's script:
d3392f4eae/tools/gen_sdk_package.sh
The location within the SDK where we place the libc++ headers is chosen
such that clang's search path detection logic for sysroots would pick up
the headers properly.
We also document this change.
