Update configure.ac

Initially I thought the --enable-addons-with-dependencies in our configure line would get picked up here since we have the dependencies required. Since it doesn't I decided to force it here and the result is MythTV PVR & IPTV Simple PVR get compiled/built. Needs testing though.

TODO: Bring PVR-Addons into the project via ExternalProject_Add, since that would avoid the need to the above and automate the updating process.