Option to use system, instead of built-in, expat

Option to use system, instead of built-in, expat

At least one Linux distribution packager of FG has complained that we're bundling expat in our binaries. To keep them happy, add the option (-DSYSTEM_EXPAT=1 at cmake time) to use the system expat instead.

For the moment (and simplicity), this requires building SIMGEAR_SHARED (which is fired to on by selecting the option).