In legacy/trac#32495, we discovered that `-Wtypedef-redefinition` doesn't seem to work on clang unless `-std=gnu99` or similar is used. (typedef redefinitions are apparently allowed in C11) It does seem to work on FreeBSD's cc for some reason, but we don't use FreeBSD in Travis. We should consider using something like `-std=gnu99` in our Travis configuration for clang, so we can catch additional instances of this sort of portability problem earlier.