Darwin / macOS should build / link to libjodycode.3.1.dylib, not libjodycode.so.3

[badger200]

The way Darwin's shared library naming scheme works is, library.version.dylib.

Both libjodycode and jdupes should have changes to respect under If Darwin, link should be libjodycode.3.1.dylib, and create symlinks libjodycode.3.dylib and libjodycode.dylib. The link for the dylib should also issue arg -install_name /usr/lib/libjodycode.3.1.dylib or whatever. I had to use install_name_tool /usr/lib/libjodycode.3.1.dylib /usr/lib/libjodycode.3.1.dylib to accomplish it manually.

Currently the libjodycode Makefile checks for Darwin SO_NAME = .dylib but then ignores it anyway and builds libjodycode.so.3.