Don't search for protoc ourselves.

FindProtobuf.cmake already searches for the protoc compiler, so there's
no need to do a custom find_file search here.
Remove custom do_protoc target.

CMake already makes ore-infinium depend on the protoc add_custom_command
because the command's OUTPUT matches a source file of the executable,
so there's no need for add_custom_target and add_dependencies.
Use PROTOBUF_GENERATE_CPP to run protoc.

Instead of running protoc with a manual add_custom_command,
use the PROTOBUF_GENERATE_CPP macro provided in FindProtobuf.