package/agentpp: fix build with gcc 4.8

Fix the following build failure with gcc 4.8 raised since bump of snmppp
to version 3.5.0 in commit e011fa0415:

configure: error: Cannot find suitable libsnmp++ library

[...]

configure:9496: checking if libsnmp++ can be linked with flags from pkg-config
configure:9528: /home/buildroot/autobuild/run/instance-1/output-1/host/bin/arm-none-linux-gnueabi-g++ -o conftest -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64  -Os -g0 -D_FORTIFY_SOURCE=1 -pthread -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -D_GNU_SOURCE -D_XOPEN_SOURCE=XPG6  conftest.cpp -L/home/buildroot/autobuild/run/instance-1/output-1/host/bin/../arm-buildroot-linux-gnueabi/sysroot/usr/lib -lsnmp++ >&5
In file included from /home/buildroot/autobuild/run/instance-1/output-1/host/arm-buildroot-linux-gnueabi/sysroot/usr/include/snmp_pp/snmp_pp.h:71:0,
                 from conftest.cpp:92:
/home/buildroot/autobuild/run/instance-1/output-1/host/arm-buildroot-linux-gnueabi/sysroot/usr/include/snmp_pp/uxsnmp.h:628:35: error: 'nullptr' was not declared in this scope
      CSNMPMessage *snmp_message = nullptr);
                                   ^

Fixes:
 - http://autobuild.buildroot.org/results/f272473e7b588f5390b183072935a0217290ee4e

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Tested-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Reviewed-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
(cherry picked from commit 7c6c018ad4)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/agentpp/agentpp.mk

@@ -11,6 +11,7 @@ AGENTPP_LICENSE = Apache-2.0
 AGENTPP_LICENSE_FILES = LICENSE-2_0.txt
 AGENTPP_INSTALL_STAGING = YES
 AGENTPP_DEPENDENCIES = host-pkgconf snmppp
+AGENTPP_CONF_ENV = CXXFLAGS="$(TARGET_CXXFLAGS) -std=c++11"
 AGENTPP_CONF_OPTS += \
 	--disable-proxy \
 	--disable-forwarder \