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
shared_module() to ensure that plugins continue to be called libfoo.so on macOS.
New GModule API will eventually be added to address this.