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65a5010e25
Fixes #1873: previously any run of `make` caused the `postgres-v15-headers` target to build. It copied a bunch of headers via `install -C`. Unfortunately, some origins were symlinks in the `./pg_install/build` directory pointing inside `./vendor/postgres-v15` (e.g. `pg_config_os.h` pointing to `linux.h`). GNU coreutils' `install` ignores the `-C` key for non-regular files and always overwrites the destination if the origin is a symlink. That in turn made Cargo rebuild the `postgres_ffi` crate and all its dependencies because it thinks that Postgres headers changed, even if they did not. That was slow. Now we use a custom script that wraps the `install` program. It handles one specific case and makes sure individual headers are never copied if their content did not change. Hence, `postgres_ffi` is not rebuilt unless there were some changes to the C code. One may still have slow incremental single-threaded builds because Postgres Makefiles spawn about 2800 sub-makes even if no files have been changed. A no-op build takes "only" 3-4 seconds on my machine now when run with `-j30`, and 20 seconds when run with `-j1`.