ld warning in macOS Sequoia

When linking a Haskell program I get the following warning

ld: warning: -U option is redundant when using -undefined dynamic_lookup

This has been true ever since I updated to the last macOS 15.

This breaks running the testsuite locally because the output has this added line. It should also be a problem when CI is updated, and is not a message we want to display to users (nothing's wrong here, we're just passing a flag unnecessarily).

We should add a check to ./configure and ghc-toolchain on whether this warning is raised and pass -U accordingly.