Simulator stops working after installing FDB 6

When fdbserver is running, the simulator stops working. This is problematic if someone wants to develop and test on OS X and I stumbled on this while working on #1058.

This is how this problem can be reproduced:

1. Download and install FDB 6 from web page.
2. run a simulation tests.

The output that I got looked roughly like this:
```
/Users/mpilman/Projects/builddirs/foundationdb/bin/fdbserver -r simulation -s 0x2582dd31 -f fast/AtomicBackupCorrectness.txt
Random seed is 629333297...
ERROR: Could not locate shared memory - 'machineId'
```

Even stopping foundationdb does not make the error go away. The only way I could figure out to make the error go away was:

1. uninstall foundationdb with the script
2. restart the machine.

As mentioned in #1058, cmake will try to find an installed foundationdb and warn the user if it can't find one (for running upgrade restart tests with an older binary). But if an installed fdb results in `make test` not working, this is generally problematic.

Start with fdbserver in simulation mode and the make test path described in the report; review how CMake finds an installed FoundationDB and run fast/AtomicBackupCorrectness.txt on OS X with FDB 6 installed. Done means simulation tests run successfully without uninstalling FoundationDB or restarting the machine.

Written by the indexing model from the issue text.