capnp: Disable building of shared libraries

The problem with a shared library is that a binary linked against it, will encode the path it was found under, and only run on systems where the shared library is present (using same path).

The advantage is that if multiple binaries link with the same library; memory and disk space can be saved, but currently no formula depends on capnp, so the assumption must be that this formula is for developers who wish to use capnp in their own software, hence why the default should be to produce a binary that can be moved to other systems, when linking with libcapnp (installed via homebrew).