Add __attribute__((format)) to log_callback on GCC and Clang.
Add __attribute__((format)) to log_callback on GCC and Clang.

Extending this for MSVC 2015 via _Printf_format_string_ still needs to
be done.

padenot/cubeb,kinetiknz/cubeb,padenot/cubeb,kinetiknz/cubeb,kinetiknz/cubeb,padenot/cubeb