Compile user_variables plugin statically

Recompile it for embedded.
Test it for embedded.
Promote it to Gamma

debian/mariadb-server-10.2.install

@@ -56,7 +56,6 @@ usr/lib/mysql/plugin/semisync_slave.so
 usr/lib/mysql/plugin/server_audit.so
 usr/lib/mysql/plugin/simple_password_check.so
 usr/lib/mysql/plugin/sql_errlog.so
-usr/lib/mysql/plugin/user_variables.so
 usr/lib/mysql/plugin/wsrep_info.so
 usr/share/apport/package-hooks/source_mariadb-10.2.py
 usr/share/doc/mariadb-server-10.2/mysqld.sym.gz

plugin/user_variables/CMakeLists.txt

-MYSQL_ADD_PLUGIN(user_variables user_variables.cc)
+MYSQL_ADD_PLUGIN(user_variables user_variables.cc
+  DEFAULT RECOMPILE_FOR_EMBEDDED)

plugin/user_variables/mysql-test/user_variables/basic.result

@@ -7,7 +7,7 @@ PLUGIN_AUTHOR	Sergey Vojtovich
 PLUGIN_DESCRIPTION	User-defined variables
 PLUGIN_LICENSE	GPL
 LOAD_OPTION	ON
-PLUGIN_MATURITY	Alpha
+PLUGIN_MATURITY	Gamma
 SHOW CREATE TABLE INFORMATION_SCHEMA.USER_VARIABLES;
 Table	Create Table
 user_variables	CREATE TEMPORARY TABLE `user_variables` (

plugin/user_variables/mysql-test/user_variables/suite.pm

@@ -6,7 +6,8 @@ return "No USER_VARIABLES plugin" unless
   $ENV{USER_VARIABLES_SO} or
   $::mysqld_variables{'user-variables'} eq "ON";
 
-return "Not run for embedded server" if $::opt_embedded_server;
+return "Not run for embedded server" if $::opt_embedded_server and
+                                        $ENV{USER_VARIABLES_SO};
 
 sub is_default { 1 }
 

plugin/user_variables/user_variables.cc

@@ -134,6 +134,6 @@ maria_declare_plugin(user_variables)
   NULL,
   NULL,
   "1.0",
-  MariaDB_PLUGIN_MATURITY_ALPHA
+  MariaDB_PLUGIN_MATURITY_GAMMA
 }
 maria_declare_plugin_end;