It seems src/lt/configure hasn't been refreshed in a long time.

In Fedora we have been carrying a patch for C99 support for a while (since 2023):

https://src.fedoraproject.org/rpms/racket/blob/rawhide/f/racket-configure-c99.patch

Nothing urgent but might be good to update so others don't run into this - I assume the patch is still valid