bug#73569: Fortran tests fail on macOS 15

The CI on the libtool master branch fails on macOS 15, while it passes
on macOS 12, 13, 14. [1] The failing tests are:

   testsuite: 47 153 154 155 170 failed

It's all about Fortran tests. The error message seen in all logs is

stderr:
ld: library 'System' not found
collect2: error: ld returned 1 exit status

I don't know whether it's a bug in GCC, a bug in Homebrew, or a bug in libtool.

Bruno

[1] https://github.com/gnu-libtool/ci-check/actions/runs/11109274264