MDEV-35421 - main.mysql_upgrade fails without unix_socket plugin

Fixed main.mysql_upgrade to pass when unix_socket plugin is unavailable.

Also don't redefine _GNU_SOURCE, which was previously defined by command
line/environment. This fixes silent auth_socket build failure with
MYSQL_MAINTAINER_MODE=ERR.

mysql-test/main/mysql_upgrade.result

@@ -1062,7 +1062,7 @@ set global sql_safe_updates=@orig_sql_safe_updates;
 # MDEV-32043 Remove plugins previously external that are now built in (unix_socket)
 #
 INSERT INTO mysql.plugin SELECT 'unix_socket', 'auth_socket.so'
-    FROM dual WHERE convert(@@version_compile_os using latin1) not in ('Win32', 'Win64', 'Windows');
+    FROM information_schema.plugins WHERE plugin_name='unix_socket' AND plugin_library IS NULL;
 # mariadb-upgrade --force --silent 2>&1
 SELECT * FROM mysql.plugin WHERE name='unix_socket';
 name	dl

mysql-test/main/mysql_upgrade.test

@@ -500,7 +500,7 @@ set global sql_safe_updates=@orig_sql_safe_updates;
 --echo #
 
 INSERT INTO mysql.plugin SELECT 'unix_socket', 'auth_socket.so'
-    FROM dual WHERE convert(@@version_compile_os using latin1) not in ('Win32', 'Win64', 'Windows');
+    FROM information_schema.plugins WHERE plugin_name='unix_socket' AND plugin_library IS NULL;
 --echo # mariadb-upgrade --force --silent 2>&1
 --exec $MYSQL_UPGRADE --force --silent 2>&1
 SELECT * FROM mysql.plugin WHERE name='unix_socket';

plugin/auth_socket/CMakeLists.txt

@@ -15,7 +15,9 @@
 # Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1335  USA
 
 CHECK_CXX_SOURCE_COMPILES(
-"#define _GNU_SOURCE
+"#ifndef _GNU_SOURCE
+#define _GNU_SOURCE
+#endif
 #include <sys/socket.h>
 int main() {
   struct ucred cred;