build c++ docs in correct location

There's two problems with the c++ documentation build: 1) after the code reorg, doxygen was not finding the correct path for the headers, and 2) it was not copying the html files to the same path that the top level avro index.html expected to find them.

This patch fixes both.

It also has 2 minor changes that were in my repo but never submitted: 1) fix a coverity warning about checking unsigned value with <0, and 2) add a #define for EOF because builds on some linux distributions were not finding it.

I'd like to get these submitted for 1.3.