Bug #51938 plugin_dir gets bad default value

When building the script directory using a CMake-based build, both the
variables in config.h.cmake (including PLUGINDIR) and the variables in
CMakeList.txt (which includes pkgplugindir).

However, for autotools-based builds, only pkgplugindir is substituted,
which means that the plugin-path is not substituted.

This patch solves the problem by using pkgplugindir, which works on both
CMake-based and autotools-based builds, instead of PLUGINDIR.

scripts/mysqld_safe.sh

@@ -699,7 +699,7 @@ fi
 
 cmd="`mysqld_ld_preload_text`$NOHUP_NICENESS"
 
-plugin_dir="${PLUGIN_DIR:-@PLUGINDIR@}${PLUGIN_VARIANT}"
+plugin_dir="${PLUGIN_DIR:-@pkgplugindir@}${PLUGIN_VARIANT}"
 
 for i in  "$ledir/$MYSQLD" "$defaults" "--basedir=$MY_BASEDIR_VERSION" \
   "--datadir=$DATADIR" "--plugin-dir=$plugin_dir" "$USER_OPTION"