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_nacm.c b/src/plugins_exts_nacm.c
index 5002e6f..ed670a8 100644
--- a/src/plugins_exts_nacm.c
+++ b/src/plugins_exts_nacm.c
@@ -19,12 +19,6 @@
 #include "libyang.h"
 #include "plugins_exts.h"
 
-/**
- * @brief Storage for ID used to check plugin API version compatibility.
- * Ignored here in the internal plugin.
-LYEXT_VERSION_CHECK
- */
-
 struct nacm_dfs_arg {
     struct lysc_ext_instance *c_ext;
     struct lysc_node *parent;
@@ -151,6 +145,10 @@
 
 /**
  * @brief Plugin descriptions for the NACM's default-deny-write and default-deny-all extensions
+ *
+ * Note that external plugins are supposed to use:
+ *
+ *   LYPLG_EXTENSIONS = {
  */
 const struct lyplg_ext_record plugins_nacm[] = {
     {