Move sqlite source to a separate location, and build with
an intermediate library target. This permits us to set compile options
easier, including an important one, NDEBUG, which improves performance.
Sqlite 3.8.0.2 also includes an updated query planner which may improve
some queries, to be tested.
Cache the parsed navigation and airport data in a binary file to reduce
startup times and memory consumption (since only referenced FGPositioned
elements are held in memory).
Data will be reimported when the mod-time of any input file is changed.
If a global file is changed (nav.dat, awy.dat, apt.dat, etc), the cache
will be completely rebuilt, which takes approximately 30 seconds on
moderate hardware. (Future work may reduce this).
