Description of the desired feature

Although FreeBSD is not as popular as Linux/macOS/Windows, there are still some FreeBSD + GMT users (see https://forum.generic-mapping-tools.org/t/building-gmt-on-freebsd-12-1-release/967), and they're also potential PyGMT users.

As I understand it, sys.platform will return freebsdX on FreeBSD X, thus function clib_names will report an error:

Operating system "freebsdx" not supported.

What we can do is:

1. Confirm the return value of sys.platform on FreeBSD (freebsdX if I understand the docs correctly)
2. Confirm the suffix of shared libraries on FreeBSD (.so based on search results)
3. Add an if test for freebsd

For points 1 and 2, we may ask for some help in this post https://forum.generic-mapping-tools.org/t/967.

We won't have a CI job testing FreeBSD, but at least we can provide some basic support.

Are you willing to help implement and maintain this feature? Yes, but we need more inputs from FreeBSD users.