plugins CHANGE define supporting macro for external plugins

Define single (one for type plugins and another for extension
plugins) macro to use in external plugins implementation declaring all
the expected variables for successful loading of the dynamic plugin.
diff --git a/src/plugins_exts_yangdata.c b/src/plugins_exts_yangdata.c
index 890266d..0e921a7 100644
--- a/src/plugins_exts_yangdata.c
+++ b/src/plugins_exts_yangdata.c
@@ -20,12 +20,6 @@
 #include "plugins_exts.h"
 
 /**
- * @brief Storage for ID used to check plugin API version compatibility.
- * Ignored here in the internal plugin.
-LYEXT_VERSION_CHECK
- */
-
-/**
  * @brief Free yang-data extension instances' data.
  *
  * Implementation of ::lyext_clb_free callback set as lyext_plugin::free.
@@ -165,6 +159,10 @@
 
 /**
  * @brief Plugin descriptions for the yang-data extension
+ *
+ * Note that external plugins are supposed to use:
+ *
+ *   LYPLG_EXTENSIONS = {
  */
 const struct lyplg_ext_record plugins_yangdata[] = {
     {