runtime-c-api should not copy files to manifest directory

[MarkMcCaskey]

cargo publish is unhappy with modifications happening outside of OUT_DIR when running build.rs.

[bjfish]

I'm guessing the current build must changing the last modified time of the file and complaining about that since the contents shouldn't change.

I wonder if we could configure the header generator not to modify the file if the contents have not changed.

Link to rust feature:
rust-lang/cargo#5584

[Hywan]

@MarkMcCaskey I believe we can close this issue now. Feel free to reopen if you think it's still relevant somehow.