Hi Tamir,
Sorry for the slow response, your e-mail for some reason landed in my spam folder.
The links you provided seem to be older forks of c-capnproto, found here:
However, that version has also been idle for 2 years.
There's no reason to think that the code won't work anymore -- Cap'n Proto changes are designed to be backwards-compatible. However, it might not support features added in the last few releases. Also, it looks like a security warning was added recently saying that the library does no bounds checking... that's pretty disappointing. :/
It would be cool to see someone revive this work or create a new C implementation.
-Kenton