utils: prefer shlib when looking through unixodbc installations

[detule]

Fixes #919

Tested by:

• Reviewed output of locate_install_unixodbc (on a mac) when both static and dynamically built unixodbc libraries were present, versus when only static were found.
• Ran new unit test on a mac.

There is an argument perhaps that we should just revert https://github.com/r-dbi/odbc/commit/aba5578e05a9e5c7b4cf3ec03d489c87de5fb348 since it didn't fix the CRAN issue and it's potentially causing issues for regular users. I would rather try and understand when we can expect good behavior when linking against the static lib (are issues driver specific, with some OEM drivers behaving better when linking against the static lib versus others?). I don't feel strongly about this though, and can go either way.