Abort build against unsupported server versions

Abort compilation immediately and obviously against unsupported server
versions.

Author: kovmir

postgresql_extension_template.c

@@ -3,6 +3,11 @@
 
 PG_MODULE_MAGIC;
 
+/* Abost compilation against unsupported versions. */
+#if PG_VERSION_NUM < 120000
+#error "Unsupported PostgreSQL Version"
+#endif
+
 Datum my_function(PG_FUNCTION_ARGS);
 PG_FUNCTION_INFO_V1(my_function);