macOS /w Xcode 15: force the old linker when targeting macOS 11 or lower

macOS /w Xcode 15: force the old linker when targeting macOS 11 or lower

the new linker has problems with weak symbols and causes runtime crashes when
trying to run those builds on older versions of macOS

https://developer.apple.com/documentation/xcode-release-notes/xcode-15-release-notes#Linking

they write the classic linker will be removed in a future release, so have it
error out for Xcode 16 for now just in case. The brave soul who tries it out
first will have to disable that AC_MSG_ERROR… (or expliticly set macOS 12 as
min target)

Change-Id: I653e5bce70b3eae0f512be11dc7e07ffc98e9812
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/157336
Tested-by: Jenkins
Reviewed-by: Patrick Luby <plubius@neooffice.org>
Reviewed-by: Christian Lohmaier <lohmaier+LibreOffice@googlemail.com>
(cherry picked from commit 809733714b7501490c5a6fa690d254ec7f9bc8f9)
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/157540
Reviewed-by: Michael Stahl <michael.stahl@allotropia.de>