When building Cap'n Proto with compilers which can't yet build the command
line tools (MSVC), the exported targets capnp_tool and capnpc_cpp don't
exist, thus attempting to use them in a generator expression when setting
CAPNP_EXECUTABLE and CAPNPC_CXX_EXECUTABLE causes a hard error.

Leaving the variables unset provides users the opportunity to set
CAPNP_EXECUTABLE and CAPNPC_CXX_EXECUTABLE manually in CMake projects
consuming Cap'n Proto as a CMake package, and produces a less mystifying
error message if the user forgets.

Thanks to Philip Quinn for advice on this issue.