configure: add check for libidn
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).

xkfz007/vlc,jomanmuk/vlc-2.2,krichter722/vlc,shyamalschandra/vlc,vlc-mirror/vlc,jomanmuk/vlc-2.2,vlc-mirror/vlc,vlc-mirror/vlc,shyamalschandra/vlc,xkfz007/vlc,vlc-mirror/vlc,krichter722/vlc,shyamalschandra/vlc,vlc-mirror/vlc,jomanmuk/vlc-2.1,krichter722/vlc,vlc-mirror/vlc-2.1,shyamalschandra/vlc,vlc-mirror/vlc-2.1,jomanmuk/vlc-2.1,vlc-mirror/vlc-2.1,jomanmuk/vlc-2.1,krichter722/vlc,jomanmuk/vlc-2.1,krichter722/vlc,vlc-mirror/vlc-2.1,shyamalschandra/vlc,vlc-mirror/vlc-2.1,xkfz007/vlc,xkfz007/vlc,jomanmuk/vlc-2.2,vlc-mirror/vlc-2.1,jomanmuk/vlc-2.1,xkfz007/vlc,jomanmuk/vlc-2.1,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,xkfz007/vlc,vlc-mirror/vlc,vlc-mirror/vlc,shyamalschandra/vlc,jomanmuk/vlc-2.2,vlc-mirror/vlc-2.1,krichter722/vlc,krichter722/vlc,jomanmuk/vlc-2.1,xkfz007/vlc,jomanmuk/vlc-2.2,shyamalschandra/vlc