Use pkglibdir instead of plugindir
Use pkglibdir instead of plugindir

Because mysql/plugin directory under /usr/lib or /usr/lib64
is not guranteed to exist.

[groonga-dev,01335]

Reported by WING. Thanks!!!
Patch by yoku ts. Thanks!!!

yoku0825/mroonga,mroonga/mroonga,yoku0825/mroonga,naoa/mroonga,yoku0825/mroonga,mroonga/mroonga,yoku0825/mroonga,mroonga/mroonga,mroonga/mroonga,naoa/mroonga,yoku0825/mroonga,yoku0825/mroonga,naoa/mroonga,naoa/mroonga,naoa/mroonga,naoa/mroonga