Try making our configure.ac script build with AC 2.70.
In versions <=2.69, according to the autoconf docs, AC_PROG_CC_C99 is needed with some compilers, if they require extra arguments to build C99 programs. In versions >=2.70, AC_PROG_CC checks for these compilers automatically, and so the AC_PROG_CC_C99 macro is obsolete.
So, what can you do if you want your script to work right with both autoconf versions? IIUC, neither including AC_PROG_CC_C99 macro nor leaving it out will give you the right behavior with both versions. It looks like you need to look at the autoconf version explicitly.
(Now, the autoconf manual implies that it's "against autoconf philosophy" to look at the autoconf version rather than trying the behavior to see if it works, but they don't actually tell you how to detect recoverably at autoconf-time whether a macro is obsolete or not, and I can't find a way to do that.)
So, is it safe to use m4_version_prereq, like I do here? It isn't listed in the autoconf 2.63 manual (which is the oldest version we support). But a mailing list message  (which added the documentation back in 2008) implies that m4_version_prereq has been there since "at least back to autoconf 2.59".
So I think this will work.
I am basing this patch against Tor 0.3.5 since, if autoconf 2.70 becomes widespread before 0.3.5 is unsupported, we might need this patch to continue 0.3.5 development. But I don't think we should backport farther than 0.4.5 until/unless that actually happens.
This is part of a fix for #40355.