I got an error while rebuilding a module for 3.4. This was a ISO C90 error but setup.py explicitely adds "-std=c99" to the gcc parameters, and indeed it is used. 

fifo.h:114:5: error: ISO C90 forbids mixed declarations and code [-Werror=declaration-after-statement]
uint32_t ofs = fifo->write_count - fifo->write_offset; 

However, Py 3.4 seems to add -Werror=declaration-after-statement also for extension modules. This should not happe (said also Yhg1s in #python).

Attached is a file that shows the setup.py and also the error log.