Tor 0.4.7.3-alpha fails to compile on old gcc

  CC       src/core/or/congestion_control_common.o
In file included from src/core/or/congestion_control_common.c:24:
./src/core/or/congestion_control_st.h:212: error: redefinition of typedef ‘congestion_control_t’
./src/core/or/congestion_control_common.h:15: note: previous declaration of ‘congestion_control_t’ was here
make[1]: *** [src/core/or/congestion_control_common.o] Error 1

This looks like a similar bug to #30184 (closed)

(We could choose to not care about old gccs, but in our past experience, if we don't fix the known issues as we see them, they'll pop up again and again on random linux distros.)