Please establish which C standard tor code complies with
Currently, tor is built without the -std=xx flag, and it isn't clear what C standard is compiler expected to use.
Please add one of -std=c90, -std=c99, -std=c11, -std=c17 to compilation lines, add the corresponding macro to configure.ac:
AX_CHECK_COMPILE_FLAG([-std=cNN], [CFLAGS="$CFLAGS -std=cNN"])
Reason#1: in FreeBSD we have several architectures some of which use different compilers (gcc/clang/etc) It is good to take guessing from the process and establish what standard the code complies with.
Reason#2: Some compilers might default to the older standard where a newer standard is required.
My guess is that the standard is c11.