MDEV-11088 Client plugins cannot be loaded by command line tools

in default installation.

Added plugin-dir to the [client] section of the generated my.ini,
so that  installed services (MSI or mysql_install_db.exe) would be able to
find plugin directory.

sql/CMakeLists.txt

@@ -387,6 +387,7 @@ IF(WIN32)
     ${CMAKE_CURRENT_BINARY_DIR}/mysql_bootstrap_sql.c
     COMPONENT Server
   )
+  SET_TARGET_PROPERTIES(mysql_install_db PROPERTIES COMPILE_FLAGS -DINSTALL_PLUGINDIR=${INSTALL_PLUGINDIR})
   TARGET_LINK_LIBRARIES(mysql_install_db mysys)
 
   ADD_LIBRARY(winservice STATIC winservice.c)

sql/mysql_install_db.cc

@@ -233,6 +233,20 @@ static void get_basedir(char *basedir, int size, const char *mysqld_path)
   }
 }
 
+#define STR(s) _STR(s)
+#define _STR(s) #s
+
+static char *get_plugindir()
+{
+  static char plugin_dir[2*MAX_PATH];
+  get_basedir(plugin_dir, sizeof(plugin_dir), mysqld_path);
+  strcat(plugin_dir, "/" STR(INSTALL_PLUGINDIR));
+
+  if (access(plugin_dir, 0) == 0)
+    return plugin_dir;
+
+  return NULL;
+}
 
 /**
   Allocate and initialize command line for mysqld --bootstrap.
@@ -313,6 +327,10 @@ static int create_myini()
     fprintf(myini,"protocol=pipe\n");
   else if (opt_port)
     fprintf(myini,"port=%d\n",opt_port);
+
+  char *plugin_dir = get_plugindir();
+  if (plugin_dir)
+    fprintf(myini, "plugin-dir=%s\n", plugin_dir);
   fclose(myini);
   return 0;
 }