Fix shared library build on macOS

[mkoeppe]

macOS needs -install_name in the linker command for the dylib.

Text for release notes

see title

Further details

see sagemath/sage#38169

[fingolfin]

Thanks for the contribution @mkoeppe, I agree this goes in the right direction. Unfortunately this can't be merged as-is, as it breaks make testlibgap on macOS (and the fact that CI passes anyway highlights that we need to make sure at least one macOS runner also runs testlibgap). Indeed, the specified -install_name is only valid for an installed .dylib, but not for when GAP is run out of its source directory, as still is the norm for many (most?) GAP users. And make testlibgap tests exactly in that scenario. It shouldn't be too hard to resolve that, though -- we already build a separate GAP binary for installation (via the build/gap-install target) so we could do something similar for libgap.