rubyPackages.pg: use pkg-config instead of pg_config

This change forces pg native extension to use pkg-config to locate libpq
instead of running pg_config. It also removes an unnecessary reference
to postgresql package since build flags are embedded in the output at
`$out/${ruby.gemPath}/build_info/*`.

pkgs/development/ruby-modules/gem-config/default.nix

@@ -573,9 +573,14 @@ in
   };
 
   pg = attrs: {
-    buildFlags = [
-      "--with-pg-config=${postgresql}/bin/pg_config"
-    ];
+    # Force pkg-config lookup for libpq.
+    # See https://github.com/ged/ruby-pg/blob/6629dec6656f7ca27619e4675b45225d9e422112/ext/extconf.rb#L34-L55
+    #
+    # Note that setting --with-pg-config=${postgresql}/bin/pg_config would add
+    # an unnecessary reference to the entire postgresql package.
+    buildFlags = [ "--with-pg-config=ignore" ];
+    nativeBuildInputs = [ pkg-config ];
+    buildInputs = [ postgresql ];
   };
 
   psych = attrs: {