Description of proposed changes

This PR provides basic support for FreeBSD.

For FreeBSD, sys.platform returns freebsdX, and the suffix of shared libraries is .so.

As per the feedback in the forum https://forum.generic-mapping-tools.org/t/building-gmt-on-freebsd-12-1-release/967/20. FreeBSD users can import pygmt correctly using this PR.

Fixes #698.

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.

Notes

• You can write /format in the first line of a comment to lint the code automatically