consider clang -std=gnu99 in Travis for better C99 portability

In #32495 (moved), 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.