Stop building our own libgc

[straight-shoota]

Originally, we built and distributed our own version of libgc because Crystal required some patches.
Those patches are long gone (#141) and we're now simply building the upstream version of libgc.

This shouldn't be our responsibility. At this point, I think we can safely drop this dependency. Distribution packages need to make sure to install the libgc package, as they already do with other dependencies.

[straight-shoota]

Some older but still commonly used distributions (such as Ubuntu 22.04, Debian 11) have quite dated versions of libgc (see https://repology.org/project/boehm-gc/versions).
On these distributions, we cannot simply use the system package because it lacks support for some features that Crystal needs.
So I guess we'll have to continue distributing a custom build of libgc.

We may consider removing it from redistributed packages (such as on OBS) when an up-to-date package is available.