Make sure generated code compiles cleanly even with pedantic warnings, since some people enable those.

Warn when an optimized build doesn't set NDEBUG and then #define it ourselves -- unless the build explicitly sets DEBUG.

Split libcapnp into three libraries:  libkj, libcapnp, and libcapnpc.  The third contains the schema parser.  Unfortunately it is rather enormous so we should give people the option to not link it.