I've just read scheme standard. Scheme has to implement both tail-rec and call/cc.
As for C, I think it's possible to implement proper tail recursion with setjmp/longjmp, but I'm not sure. Most C compilers have tail call optimizations I think, not because it's so simple but because it's so important.
BTW, if perl is implemented like this (in a stackless way) then it may be simple to implement tail-recursion optimization (but the developpers must have some reason why it's not already done). Maybe it's also possible to create true call/cc: reference counting should be enough for most uses of call/cc, like exceptions and multithreading.