postgresql: improve fake pg_config in default output

This fixes some build systems which look up the location of pg_config
via the location of the postgres binary itself, e.g. timescaledb,
instead of calling pg_config which is on PATH.

Since the -dev output is correctly placed before the default output of
postgresql in PATH, we can rely on that and call "pg_config" from the
default output's fake script. Only do that, when the one on PATH is
actually a different file, though, to prevent infinite loops.

Resolves #341408

pkgs/servers/sql/postgresql/generic.nix

@@ -188,10 +188,17 @@ let
         moveToOutput "bin/pg_config" "$dev"
         # To prevent a "pg_config: could not find own program executable" error, we fake
         # pg_config in the default output.
-        cat << EOF > "$out/bin/pg_config" && chmod +x "$out/bin/pg_config"
+        cat << 'EOF' > "$out/bin/pg_config" && chmod +x "$out/bin/pg_config"
         #!${stdenv'.shell}
-        echo The real pg_config can be found in the -dev output.
-        exit 1
+        my_path="$(readlink -f -- "$0")"
+        on_path="$(readlink -f -- "$(which pg_config)")"
+        if [[ "$my_path" == "$on_path" ]]; then
+          echo The real pg_config can be found in the -dev output.
+          exit 1
+        else
+          # There is a different pg_config in PATH, let's call this instead.
+          pg_config "$@"
+        fi
         EOF
         wrapProgram "$dev/bin/pg_config" --argv0 "$out/bin/pg_config"