mosh: fix build with protobuf 3.6.0

protobuf now requires C++11

Fixes:
 - http://autobuild.buildroot.net/results/6e4550e080a9d558b441dcc735b6be14f39fb03f

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/mosh/mosh.mk

@@ -10,6 +10,9 @@ MOSH_DEPENDENCIES = zlib ncurses protobuf host-pkgconf
 MOSH_LICENSE = GPL-3.0+ with exception
 MOSH_LICENSE_FILES = COPYING COPYING.iOS
 
+# protobuf needs c++11 (since 3.6.0)
+MOSH_CONF_ENV += CXXFLAGS="$(TARGET_CXXFLAGS) -std=c++11"
+
 ifeq ($(BR2_PACKAGE_OPENSSL),y)
 MOSH_CONF_OPTS += --with-crypto-library=openssl
 MOSH_DEPENDENCIES += openssl