Currently, both the "bsoncxx" and "bsoncxx_static" targets depend on the "bson-1.0" library, which is the shared libbson library.

We should remove the "bsoncxx_static" target, and make the "bsoncxx" target a shared or static library depending on the user value for BUILD_SHARED_LIBS. If BUILD_SHARED_LIBS is on, "bsoncxx" should depend on "bson-1.0"; else, if should depend on "bson-static-1.0".

This has the effect of limiting users to building either the static library or the shared library, but not both at once.