It looks like -Werror=declaration-after-statement was added to BASECFLAGS in configure.ac during the 3.4 development cycle by a3559c8c614b and e47806951fb2. Unfortunately, BASECFLAGS propagates through to extension module builds as well.  If -Werror=declaration-after-statement should only be restricted to the build of the interpreter executable itself, one option *might be* to move the test and definition to CFLAGSFORSHARED in configure.ac.

A workaround could be to define CFLAGS before rebuilding a module:

export CFLAGS=$(python3.4 -c 'import sysconfig; print(sysconfig.get_config_var("CFLAGS").replace("-Werror=declaration-after-statement",""))')

(As usual, my brain hurts after trying to sift through the myriad build flags and their interactions in configure.ac, Makefile, and setup.py.)