Protobuf: Fix generated source paths with PROTOBUF_GENERATE_CPP_APPEND_PATH OFF
Currently SRC/HDRS reference non-existing files, given PROTOBUF_GENERATE_CPP_APPEND_PATH is set to OFF.
This properly fixes !2212 (merged) (commit 71d99e15); protobuf does maintain the directory structure relative to the closest include path.
When the PROTOBUF_GENERATE_CPP_APPEND_PATH=OFF option is used, the include path is the sources root. Thus all generated files will be on a path in the output directory, matching the path relative to the sources directory.
Topic-rename: FindProtobuf-gen-sources