Fixed Makefile for FreeBSD

[vazub]

FreeBSD puts all non-base system files under /usr/local prefix, which results in <unwind.h> not being found (with libunwind being actually installed) when compiling with runtime-bitcode=yes option, unless proper CPATH and LIBRARY_PATH environment variables are defined. This PR addresses the issue, by providing relevant checks and adjustments in the Makefile.

[ponylang-main]

Hi @vazub,

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 3808.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]

Thanks @vazub.