Set the library dir, and use that to find plugins

The plugins might be in lib or lib64 depending on the build settings.

Change-Id: Ia6830f7f2612ff59cc2f6bceb0edc9621ba9eeb5
Added missing AvogadroRendering link

This is missed when building static plugins, but needed for dynamic
plugins.

Change-Id: Ib5584cd13e6c9f56731e49f8c0a884c354fd843e
Merge branch 'plugins'

Change-Id: I51db9a50b20c31d7c3600e71633d4f00b2a7e909