libxslt: fix cygwin libtool error

pkgs/development/libraries/libxslt/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchurl, fetchpatch, libxml2, findXMLCatalogs, python2
+{ stdenv, hostPlatform, fetchurl, fetchpatch, libxml2, findXMLCatalogs, python2
 , cryptoSupport ? false
 , pythonSupport ? (! stdenv ? cross)
 }:
@@ -18,6 +18,12 @@ stdenv.mkDerivation rec {
 
   patches = stdenv.lib.optional stdenv.isSunOS ./patch-ah.patch;
 
+  # fixes: can't build x86_64-unknown-cygwin shared library unless -no-undefined is specified
+  postPatch = optionalString hostPlatform.isCygwin ''
+    substituteInPlace tests/plugins/Makefile.in \
+      --replace 'la_LDFLAGS =' 'la_LDFLAGS = $(WIN32_EXTRA_LDFLAGS)'
+  '';
+
   outputs = [ "bin" "dev" "out" "doc" ] ++ stdenv.lib.optional pythonSupport "py";
 
   buildInputs = [ libxml2.dev ] ++ stdenv.lib.optionals pythonSupport [ libxml2.py python2 ];