I got following requirements,
1. less than 10k, better in single or two source files
2. logging by priorities
3. multi-threading safe
4. cross-platform, in c or c++
There are plenty. Ezlogger suits you best given your requirements. It is small enough and distributed as bunch of hpp files.
Boost log maybe what you are looking for, although I do not know of its status as part of the Boost suite.