Fix build on MacOS.

* configure.ac (HAVE_DARWIN_SYSTEM): New conditional.
* src/Makefile.am: On MacOS, do not use the -module flag and name
the library libscute instead of scute.
* tests/Makefile.am: Link against libscute accordingly.

On MacOS, having the library named 'scute' instead of 'libscute'
had the side effect of leading Libtool to produce a bundle (.so)
instead of the expected shared library (.dylib).

• GnuPG-bug-id: T4703
• Signed-off-by: Damien Goutte-Gattat <dgouttegattat@incenp.org>