Allow for building on DragonFlyBSD again

[mneumann]

DragonFly is a BSD, but it is not reflected in the output of uname
which returns just DragonFly and NOT DragonFlyBSD.

[SeanTAllen]

@mneumann I assume that ponyc still builds on dragonfly once these changes are in made, correct?

[ponylang-main]

Hi @mneumann,

The changelog - fixed label was added to this pull request; all PRs with a changelog label need to have release notes included as part of the PR. If you haven't added release notes already, please do.

Release notes are added by creating a uniquely named file in the .release-notes directory. We suggest you call the file 3654.md to match the number of this pull request.

The basic format of the release notes (using markdown) should be:

## Title

End user description of changes, why it's important,
problems it solves etc.

If a breaking change, make sure to include 1 or more
examples what code would look like prior to this change
and how to update it to work after this change.

Thanks.

[SeanTAllen]

@mneumann for release notes something along the lines of:

## Allow for building on DragonFlyBSD again

When the ponyc build system was switched from make to cmake, the ability to build on DragonFlyBSD was inadvertenyly removed. The build system has been updated so that ponyc can be built again on DragonFly.

[SeanTAllen]

This is good except that it needs to be updated to include release notes.

[SeanTAllen]

I'm going to merge this and add release notes manually afterwards.

[mneumann]

@SeanTAllen Sorry for my late response! I just read the 0.38 announcement and saw the entry for DragonFly. Pony on DragonFly did not build easily before this change (see here for instructions), and I doubt it will build easily with this change. There are some other problems, mainly libatomic related, as our llvm does not ship with libatomic (which, btw is not needed as LLVM emits opcodes for these intrinsics directly; still it links to it). Someone on DragonFly IRC mentioned that port misc/cxx_atomics helps to solve this, but I haven't tried out since then. Let me give this another try (DragonFly box booting up...). Would be nice to be able to build pony without any further patches and to finally have it in ports like on FreeBSD and maybe even via ponyup.