Export `gst_plugin_NAME_register()` and `gst_plugin_NAME_get_desc()` functions if targetting GStreamer >= 1.14

The following discussion from gst-plugin-rs!42 should be addressed:

• @slomo started a discussion:

  With GStreamer >= 1.14 we should optionally just create a gst_plugin_NAME_get_desc() and gst_plugin_NAME_register() function like GST_PLUGIN_DEFINE does. That makes everything easier for us as we can run actual code instead of having to statically create a C struct here.

  It would also allow using actual strings instead of NUL-terminated byte arrays for the plugin metadata.