libbson has Autotools logic for generating a platform-specific version of stdint.h, called bson-stdint.h. On CMake we just always use a handwritten bson-stdint.h which just includes stdint.h. Determine whether there are platforms we care about which don't have stdint.h and if so, port the Autotools logic to CMake.

Step One: determine if we need to do anything in particular here.