libidn: fix build without makeinfo

Build fails when the makeinfo utility is not installed on the host.

Fixes:
http://autobuild.buildroot.net/results/dfd/dfdfb34ed81ba3a4b7a7271be482e75eca849dbf/
http://autobuild.buildroot.net/results/b33/b33c0b0e6b1033ab1d1294a91b869ee6adcd391a/
http://autobuild.buildroot.net/results/940/9401cc10f6da6a2e3453ebc65ce573c370733fb5/

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Tested-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
(cherry picked from commit f6227928cd55c8d2d6ddac64773367c0edde095d)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/libidn/libidn.mk

@@ -7,7 +7,7 @@
 LIBIDN_VERSION = 1.33
 LIBIDN_SITE = $(BR2_GNU_MIRROR)/libidn
 LIBIDN_INSTALL_STAGING = YES
-LIBIDN_CONF_ENV = EMACS="no"
+LIBIDN_CONF_ENV = EMACS="no" MAKEINFO=true
 LIBIDN_CONF_OPTS = --disable-java --enable-csharp=no
 LIBIDN_DEPENDENCIES = host-pkgconf $(if $(BR2_NEEDS_GETTEXT_IF_LOCALE),gettext) $(if $(BR2_PACKAGE_LIBICONV),libiconv)
 LIBIDN_LICENSE = GPLv2+, GPLv3+, LGPLv3+