One more thing: building with a recent gcc on Linux:
$ gcc --version
gcc (Debian 6.3.0-18) 6.3.0 20170516

also gives an "unused but set variable" warning.

./Modules/expat/xmlparse.c: In function ‘gather_time_entropy’:
./Modules/expat/xmlparse.c:780:7: warning: variable ‘gettimeofday_res’ set but not used [-Wunused-but-set-variable]
   int gettimeofday_res;
       ^~~~~~~~~~~~~~~~

Since I didn't see it in time, I didn't try to fix this for 3.6.2 but it would be best to silence it going forward.