Check fails on MacOS 15

[Hans Åberg]

With gmp-6.3.0, there are 'make check' fails on MacOS 15.0, both when using MacPorts gcc14 14.2.0 and clang 18.1.8, and the tests failing are the same, 7 in number. Discussions on MacPorts ticket #70859 suggests that it is the linker that reports it:

On MacOS 14, the ld that GCC calls has an option ld_classic which is deprecated on MacOS 15, and it has therefore been removed, MacPorts ticket #70951.

https://trac.macports.org/ticket/70859
https://trac.macports.org/ticket/70951