nix: Build only libpq instead of full postgresql package

nix/libpq.nix

@@ -0,0 +1,57 @@
+# Creating a separate libpq package is is discussed in
+# https://github.com/NixOS/nixpkgs/issues/61580, but nixpkgs has not moved
+# forward, yet.
+# This package is passed to postgresql-libpq (haskell) which needs to be
+# cross-compiled to the static build and possibly other architectures as
+# as well. To reduce the number of dependencies that need to be built with
+# it, this derivation focuses on building the client libraries only. No
+# server, no tests.
+{ stdenv
+, lib
+, openssl
+, zlib
+, libkrb5
+, icu
+, postgresql
+, pkg-config
+, tzdata
+}:
+
+stdenv.mkDerivation {
+  pname = "libpq";
+  inherit (postgresql) src version;
+
+  configureFlags = [
+    "--without-gssapi"
+    "--without-readline"
+    "--with-openssl"
+    "--with-system-tzdata=${tzdata}/share/zoneinfo"
+  ];
+
+  nativeBuildInputs = [ pkg-config tzdata ];
+  buildInputs = [ openssl zlib ];
+
+  buildFlags = [ "submake-libpq" "submake-libpgport" ];
+
+  installPhase = ''
+    runHook preInstall
+
+    make -C src/bin/pg_config install
+    make -C src/common install
+    make -C src/include install
+    make -C src/interfaces/libpq install
+    make -C src/port install
+
+    rm -rfv $out/share
+
+    runHook postInstall
+  '';
+
+  outputs = [ "out" ];
+
+  meta = with lib; {
+    homepage = "https://www.postgresql.org";
+    description = "Client API library for PostgreSQL";
+    license = licenses.postgresql;
+  };
+}