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`.
