Add target-based functionality to capnp_generate_cpp() CMake function

As discussed with @pqu in #575, capnp_generate_cpp() could be improved with a target-based approach, like:

find_package(CapnProto)
add_library(foo STATIC foo.capnp)
capnp_generate_cpp(TARGET foo)
target_link_libraries(foo PUBLIC CapnProto::capnp)

This avoids the need for a call to target_include_directories() and associated worrying about where headers are generated.