By default, if the system version exists and can be used,
then it will be used. Otherwise, the local version will be used
Original patch by Saikrishna Arcot via
https://gitorious.org/fg/flightgear/merge_requests/1582
slightly modified
- Not compiled by default, nor hooked up to anything yet.
- Both libraries licensed under modified BSD license.
- Added top-level CMake file for both, to create a plain static
library for each.
