diff -up kdelibs-3.5.10/configure.in.than kdelibs-3.5.10/configure.in
--- kdelibs-3.5.10/configure.in.than	2017-06-20 18:02:22.845910412 +0200
+++ kdelibs-3.5.10/configure.in	2017-06-20 18:03:09.058172814 +0200
@@ -692,10 +692,10 @@ AC_ARG_WITH(libidn,
   [], with_libidn=check)
 LIB_IDN=
 if test "x$with_libidn" != xno; then
-  AC_CHECK_HEADERS([idna.h punycode.h stringprep.h])
-  KDE_CHECK_LIB(idn, idna_to_ascii_4i, [
+  AC_CHECK_HEADERS([idn2])
+  KDE_CHECK_LIB(idn2, idna_to_ascii_4i, [
     AC_DEFINE_UNQUOTED(HAVE_LIBIDN, 1, [Defined if you have libidn in your system])
-    LIB_IDN=-lidn
+    LIB_IDN=-lidn2
   ])
   if test "x$with_libidn" != xcheck && test -z "$LIB_IDN"; then
     AC_MSG_ERROR([--with-libidn was given, but test for libidn failed])
diff -up kdelibs-3.5.10/kdecore/configure.in.in.than kdelibs-3.5.10/kdecore/configure.in.in
--- kdelibs-3.5.10/kdecore/configure.in.in.than	2017-06-20 17:53:41.544511539 +0200
+++ kdelibs-3.5.10/kdecore/configure.in.in	2017-06-20 18:01:27.557989259 +0200
@@ -48,10 +48,10 @@ AC_ARG_WITH(libidn,
   [], with_libidn=check)
 LIB_IDN=
 if test "x$with_libidn" != xno; then
-  AC_CHECK_HEADERS([idna.h punycode.h stringprep.h])
-  KDE_CHECK_LIB(idn, idna_to_ascii_4i, [
+  AC_CHECK_HEADERS([idn2.h])
+  KDE_CHECK_LIB(idn2, idna_to_ascii_4i, [
     AC_DEFINE_UNQUOTED(HAVE_LIBIDN, 1, [Defined if you have libidn in your system])
-    LIB_IDN=-lidn
+    LIB_IDN=-lidn2
   ])
   if test "x$with_libidn" != xcheck && test -z "$LIB_IDN"; then
     AC_MSG_ERROR([--with-libidn was given, but test for libidn failed])