Signed-off-by: A. Jesse Jiryu Davis <jesse@mongodb.com>

Fix the "static declaration of 'gettid' follows non-static declaration" error when building with Android NDK r16.

bson_utf8_validate() with allow_null=false should prohibit the UTF-8
two-byte code for NULL, as well as the single-byte NULL.

After I fixed the JSON test runner on Windows it revealed that MSVC 2013
and older aren't precise enough to pass this floating-point test.

CDRIVER-2245 Add conditionals around printf defines