Fix: cmake clean issue (lib.rs.cc not found after cleaning the build directory)

[pedrolcl]

Closes #1

This issue can be triggered by the cmake clean target after a successful build. Any other build attempt after that will trigger the reported error. It doesn't matter if the build tool is GNU Make or Ninja. For instance:

cd cpp-with-rust
cmake -S . -B build
# (configuration succeeds)
cmake --build build/
# (build succeeds)
cmake --build build/ --target clean
# (clean removes two files)
cmake --build build/
# (build fails!)

Another available workaround is running cargo clean before building. It fully removes the target/ directory.

The problem is that the cmake clean target removes the OUTPUT files generated by add_custom_command(), but cargo doesn't know how to regenerate them when the target directory is already populated by the last cargo execution.

The COMMENT message added to add_custom_command() proves that cmake tries to run cargo whenever the ${BLOBSTORE_BRIDGE_CPP} file is missing. The message is printed no matter if the build fails.

My first attempt to fix this issue was adding BYPRODUCTS ${CARGO_TARGET_DIR} to add_custom_command(), which works fine if the CMake generator is GNU Make, but fails for Ninja. The alternative is to set the ADDITIONAL_CLEAN_FILES property on the main target, which emulates the cargo clean effects.