package/mutt: add optional support for libidn

When libidn was compiled before, mutt will use it as optional
dependency:

$ output/host/usr/bin/x86_64-linux-readelf -a output/target/usr/bin/mutt | grep NEEDED
 0x0000000000000001 (NEEDED)             Shared library: [libncurses.so.5]
 0x0000000000000001 (NEEDED)             Shared library: [libidn.so.11]
 0x0000000000000001 (NEEDED)             Shared library: [libintl.so.8]
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.1]

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/mutt/mutt.mk

@@ -17,6 +17,13 @@ MUTT_DEPENDENCIES += libiconv
 MUTT_CONF_OPTS += --enable-iconv
 endif
 
+ifeq ($(BR2_PACKAGE_LIBIDN),y)
+MUTT_DEPENDENCIES += libidn
+MUTT_CONF_OPTS += --with-idn
+else
+MUTT_CONF_OPTS += --without-idn
+endif
+
 ifeq ($(BR2_PACKAGE_MUTT_IMAP),y)
 MUTT_CONF_OPTS += --enable-imap
 else