configure: add check for libidn

Note that versions 1.24 and later of GNU/libidn are dual-licensed under
GPLv2+ and LGPLv3+. As with GnuTLS or Samba client, there are 3 ways
to retain LibVLC binaries under LGPLv2.1+:
 - link against a version of GNU/libidn from last year (or older),
 - do not support IDN at all,
 - use another IDNA back-end (e.g. Windows Vista's Normaliz.dll).

configure.ac

@@ -776,6 +776,17 @@ fi
 AM_CONDITIONAL(HAVE_MINIZIP, [ test "${have_minizip}" = "yes" ])
 
 
+dnl
+dnl Domain name i18n support via GNU libidn
+dnl
+PKG_CHECK_MODULES([IDN], [libidn], [
+  have_libidn="yes"
+  AC_DEFINE([HAVE_IDN], 1, [Define to 1 if you have GNU libidn.])
+], [
+  have_libidn="no"
+])
+
+
 dnl Check for dbus
 AC_ARG_ENABLE(dbus,
   [AS_HELP_STRING([--enable-dbus],