gexiv2: fix tests on darwin

pkgs/development/libraries/gexiv2/default.nix

···
 
   doCheck = true;
 
-  preCheck = ''
+  preCheck = let
+    libSuffix = if stdenv.isDarwin then "2.dylib" else "so.2";
+  in ''
     # Our gobject-introspection patches make the shared library paths absolute
     # in the GIR files. When running unit tests, the library is not yet installed,
     # though, so we need to replace the absolute path with a local one during build.
     # We are using a symlink that will be overridden during installation.
     mkdir -p $out/lib
-    ln -s $PWD/gexiv2/libgexiv2.so.2 $out/lib/libgexiv2.so.2
+    ln -s $PWD/gexiv2/libgexiv2.${libSuffix} $out/lib/libgexiv2.${libSuffix}
   '';
 
   passthru = {