This makes it a lot easier for CMake based projects to use Cap'n Proto.

Example usage:

find_package(CapnProto)
capnp_generate_cpp(FOO_SRCS FOO_HDRS foo.capnp)
add_executable(foo main.cpp ${FOO_SRCS})
target_link_libraries(foo CapnProto::capnp CapnProto::capnp-rpc)

This is a lot better than the previous variable based solution since
linking to nonexistent targets is an error whereas an empty variable
expansion (e.g. due to typos) will be silently ignored. It also makes
sure that the right compiler flags, include directories, defines and
link libraries are passed to the compiler for that target without
needing any other include_directories(), etc.