I had some strange error when trying to compile flightgear, it said it was finding simgear but the tests where failing.
My problem was that I was compiling in another partition and it was mounted as "noexec" so you could not run executables on that partition.
So, if you have problems when running cmake, one thing to check is if it is working on a partition where you can run executables. I'm sharing this because I think having FlightGear on another partition might be normal because it can take quite a lot of space.