CDRIVER-2083 set BSON_STATIC_LIBRARIES for CMake

When a CMake project links to libbson statically it should do:

find_package (libbson-static-1.0)

With this change, the above command now sets BSON_STATIC_LIBRARIES, not
BSON_LIBRARIES, so a CMake project can now link some of its targets to
libbson statically and some dynamically.

Also, define BSON_STATIC when building the libbson static library. This
avoids marking libbson's public functions for dynamic-library export
when building libbson's static lib. It probably has no effect either
way but seems like the logical thing to do.