Don't prompt for libc type when not on Linux

[SeanTAllen]

Currently, Darwin users are prompted for the libc type. This shouldn't
be happening. Only Linux users should get prompted.

Closes #178

[ponylang-main]

Hi @SeanTAllen,

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 179.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.

[Theodus]

Looks good. Thanks! Just one suggested fix for an existing typo.

[Theodus]

LGTM