I've programmed C for 35 years, and namespaces clashes have happened for me exactly once. There are two JSON libraries that use json_* as a prefix and have conflicting symbols. This actually caused a quite difficult to track down bug: https://bugzilla.redhat.com/show_bug.cgi?id=2001062

However this is not a reason to add namespaces. (In fact the bug was fixed using symbol versioning, an already existing feature of ELF.)