librttopo: fix cross compilation

pkgs/by-name/li/librttopo/package.nix

···
   nativeBuildInputs = [
     autoreconfHook
     validatePkgConfig
-    geos # for geos-config
   ];
 
   buildInputs = [ geos ];
+
+  configureFlags = [
+    "--with-geosconfig=${lib.getExe' (lib.getDev geos) "geos-config"}"
+  ];
 
   enableParallelBuilding = true;