add check for shm_open in librt and link to it if we find it

[spotrh]

In Fedora, we had a bug report that rgeolocate had stopped building. Apparently, -lrt used to be in the default linking flags inherited from R, but now it is not. This causes rgeolocate to have a missing symbol for shm_open. This pull request adds a check in configure for shm_open in librt, and passes it into the PKG_LIBS to be used at compile if found.