Let get_clib_path find the path to libgmt on Windows

[seisman]

Description of proposed changes
The name of the shared library is gmt_w64.dll for 64-bit Windows, and gmt_w32.dll for 32-bit Windows, and the library is located in C:\programs\gmt6\bin by default. This PR changes get_clib_path so that it can return the correct library name for Windows.

I've tried to install pygmt on Windows and run the test. The GMT6 binary is the one provided by Joaquim.

The tests give 19 failures. Most of the failures are due to different image size, which may be caused by slightly different ghostscript (the version I use is 9.22). Other failures seems related to the slightly different behaviors of Python packages for different system, and permission issues. See the attached log for details. pytest.log

Although currently we don't have CI tests on Windows, I think we can at least merge this PR into master, so that Windows users can have a try and help us find bugs on Windows.

Working towards #46

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.

[leouieda]

@seisman thanks for reporting these! We can merge these changes in and work towards fixing those issues in other PRs until we can get Windows working.