whois: add libidn2 support

libidn2 support is enabled since February 2017 (5.2.15):
https://github.com/rfc1036/whois/commit/8ed6d730959b1f988dd43a511c40b9419bb2b0db

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/whois/whois.mk

@@ -27,6 +27,11 @@ WHOIS_DEPENDENCIES += libidn
 WHOIS_MAKE_ENV += HAVE_LIBIDN=1
 endif
 
+ifeq ($(BR2_PACKAGE_LIBIDN2),y)
+WHOIS_DEPENDENCIES += libidn2
+WHOIS_MAKE_ENV += HAVE_LIBIDN2=1
+endif
+
 ifeq ($(BR2_SYSTEM_ENABLE_NLS),y)
 WHOIS_BUILD_TARGETS =
 WHOIS_INSTALL_TARGETS = install