Starting from ruby 3.2, warnings are raised concerning allocators

[BenTalagan]

Hi, all. Thanks to the efforts of @uwabami, @0xdevalias and info shared by others in different issue threads and pull requests, it is still possible to compile and use this gem with gsl 2.1, but as ruby is evolving and systems are updated it's getting more and more difficult to achieve this.

Upgrading a project today to ruby 3.2.x, some warnings started to appear when running the ruby app, like :

/INSTALL_PATH/lib/gsl/interp2d_fix.rb:27: warning: undefining the allocator of T_DATA class GSL::Vector

Looks like it's not a big matter and only a hint to gem developers that some requirements are not met when wrapping allocators for C class, as explained here :

https://bugs.ruby-lang.org/issues/18007

This could probably be fixed easily by making the right calls to rb_define_alloc_func() or rb_undef_alloc_func(). I don't have the knowledge (yet) to propose a proper fix or a PR, but this probably would be a good companion for #73 .