Describe the bug
As of v1.5.6, the minos and sdk settings are no longer respected. This works on version 1.5.5.

(It is possible this is an interaction with MacPorts where I am building the library.

To Reproduce
Steps to reproduce the behavior:

1. Install macports
2. echo 'macosx_deployment_target 11.0' | tee -a ${PREFIX}/etc/macports/macports.conf
echo 'macosx_sdk_version 11.3' | tee -a ${PREFIX}/etc/macports/macports.conf
3. sudo port install -sv zstd
4. look in macports lib directory and run otool -l libzstd.1.dylib
5. see output includes minos and sdk of the current OS, rather than minos 11.0 and sdk 11.3

Expected behavior
Should correctly set the OS and SDK versions in the dylib.

Screenshots and charts
If applicable, add screenshots and charts to help explain your problem.

Desktop (please complete the following information):

• OS: Mac
• Version 14.4.1
• Compiler clang
• Flags not sure
• Other relevant hardware specs not relevant
• Build system CMAKE

Additional context
Downstream bug reports:
https://trac.macports.org/ticket/69944#comment:1
https://gitlab.gnome.org/GNOME/gimp/-/issues/11483