virt-top: fix build with gettext 0.25

pkgs/applications/virtualization/virt-top/default.nix

@@ -7,6 +7,7 @@
   libxml2,
   pkg-config,
   getopt,
+  gettext,
 }:
 
 stdenv.mkDerivation rec {
@@ -43,9 +44,10 @@ stdenv.mkDerivation rec {
     ]
     ++ [ libxml2 ];
 
-  prePatch = ''
+  postPatch = ''
     substituteInPlace ocaml-dep.sh.in --replace-fail '#!/bin/bash' '#!${stdenv.shell}'
     substituteInPlace ocaml-link.sh.in --replace-fail '#!/bin/bash' '#!${stdenv.shell}'
+    substituteInPlace configure.ac --replace-fail 'AC_CONFIG_MACRO_DIR([m4])' 'AC_CONFIG_MACRO_DIRS([m4 ${gettext}/share/gettext/m4])'
   '';
 
   meta = {