meson: Always set G_MODULE_SUFFIX to `so` on macOS
meson: Always set G_MODULE_SUFFIX to `so` on macOS

This is what Autotools does, and it's what all consumers of the
GModule API expect. Without this change, people on macOS upgrading to
a GLib built with Meson will find that their plugins no longer load.

Projects that use Meson and the `g_module_build_path()` API such as
glib-networking should pass `name_suffix:` to `shared_module()` to
ensure that plugins continue to be called libfoo.so on macOS.

New GModule API will eventually be added to address this.

See also:
https://gitlab.gnome.org/GNOME/glib/issues/1413
https://gitlab.gnome.org/GNOME/glib-networking/blob/a3d81719fe7aa4a53ef49af63c19f79f09bafcdb/meson.build#L108

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib