Dynamically load GEOSPreparedContainsXY where available

[dbaston]

This is my attempt to update geos-performance so we can look at how libgeos/geos#674 affects point-in-polygon performance assuming that client code is updated to use GEOSPreparedContainsXY.

It works well for me and gives the desired "no impact on performance" answer. I don't know what you think about the complexity it adds, or how if this code even runs on MacOS. But I think this presents an alternative to the "only use functions available in the oldest version you want to test" limitation.

[pramsey]

Hm, will need to do something about file extensions, "libgeos_c.dylib" is what I've got. There is a dlopen() documented so that part might work. I will experiemnt.