capnp_generate_cpp() needs to import two directories by default: the src-prefix directory which houses the .capnp files being compiled (under their canonical paths), and the capnp include directory which houses capnp/rpc.capnp, capnp/schema.capnp, etc. (This latter path will necessarily be different for build-tree usage versus install-tree usage.) Before this commit, instead of importing the src-prefix, we were importing CMAKE_CURRENT_SOURCE_DIR.

Note that for capnproto's own compilation of test.capnp and friends, the src-prefix and capnp include directory will end up being the exact same directory. It turns out to be surprisingly difficult to de-duplicate them, however, because the capnp include directory is actually a CMake generator expression in order to deal with the build-tree versus install-tree issue mentioned above. So, for now, we pass two -I flags to the same directory.

Alongside the fix in capnp_generate_cpp(), I fixed the usage of it in c++/src/capnp/CMakeLists.txt by setting CAPNPC_SRC_PREFIX correctly and removing the /capnp suffix hack from CAPNPC_OUTPUT_DIR.