qgis3{-ltr}: change linking from MacPorts' to the System's libunwind

[nilason]

Description

QGIS does not use libunwind, but due to it's dependence to PDAL (which does use libunwind) and the following presence of libunwind in $prefix/lib, causes the MacPorts version to be picked up by the linker instead of the system's version. A thrown exception is therefore wrongfully handled by both versions, causing crash.

This is a workaround, awaiting separate prefix for the port libunwind. See https://trac.macports.org/ticket/66250#comment:1

Closes https://trac.macports.org/ticket/68250

Putting up this initially as a draft, it works for me, but this workaround probably needs to be run conditionally depending on OS version.

Type(s)

• bugfix
• enhancement
• security fix

Tested on

macOS 13.6 22G120 arm64
Xcode 15.0 15A240d

Verification

Have you

• followed our Commit Message Guidelines?
• squashed and minimized your commits?
• checked that there aren't other open pull requests for the same change?
• referenced existing tickets on Trac with full URL?
• checked your Portfile with port lint --nitpick?
• tried a full install with sudo port -vst install?
• tested basic functionality of all binary files?
• checked that the Portfile's most important variants haven't been broken?

[macportsbot]

Notifying maintainers:
@Veence for port qgis3.

[nilason]

Closing in favour of #20638, #20632 or whatever we come to agree on.