Hi John,
Sorry for the very slow response, I've been on leave.
Fibers won't be used unless you explicitly ask for them.
However, regardless of fibers, you cannot use a version of Cap'n Proto compiled with exceptions enabled inside of an application compiled with exceptions disabled. Compiling with -fno-exceptions is essentially an ABI-breaking switch for libcapnp and libkj; you must apply the same flag when building the library itself and the app that uses it.
So, unfortunately, it sounds like you cannot use the system-installed capnp, which is typically built with exceptions enabled.