package/sconeserver: fix build with icu and libxml2

Fix the following build failure raised since addition of icu support in
libxml2 through commit 49d7c85421e87d9d762be9352805cb47a2f0bbeb:

In file included from /home/peko/autobuild/instance-1/output-1/host/mips64el-buildroot-linux-gnu/sysroot/usr/include/unicode/ptypes.h:52:0,
                 from /home/peko/autobuild/instance-1/output-1/host/mips64el-buildroot-linux-gnu/sysroot/usr/include/unicode/umachine.h:46,
                 from /home/peko/autobuild/instance-1/output-1/host/mips64el-buildroot-linux-gnu/sysroot/usr/include/unicode/utypes.h:38,
                 from /home/peko/autobuild/instance-1/output-1/host/mips64el-buildroot-linux-gnu/sysroot/usr/include/unicode/ucnv_err.h:88,
                 from /home/peko/autobuild/instance-1/output-1/host/mips64el-buildroot-linux-gnu/sysroot/usr/include/unicode/ucnv.h:51,
                 from /home/peko/autobuild/instance-1/output-1/host/mips64el-buildroot-linux-gnu/sysroot/usr/bin/../../usr/include/libxml2/libxml/encoding.h:31,
                 from /home/peko/autobuild/instance-1/output-1/host/mips64el-buildroot-linux-gnu/sysroot/usr/bin/../../usr/include/libxml2/libxml/parser.h:812,
                 from conftest.cpp:61:
/home/peko/autobuild/instance-1/output-1/host/mips64el-buildroot-linux-gnu/sysroot/usr/include/unicode/localpointer.h:224:41: error: 'noexcept' does not name a type
     LocalPointer(LocalPointer<T> &&src) U_NOEXCEPT : LocalPointerBase<T>(src.ptr) {
                                         ^
/home/peko/autobuild/instance-1/output-1/host/mips64el-buildroot-linux-gnu/sysroot/usr/include/unicode/localpointer.h:224:41: note: C++11 'noexcept' only available with -std=c++11 or -std=gnu++11

Fixes:
 - http://autobuild.buildroot.org/results/be043792e6e5501631517881b10096291a654364

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/sconeserver/sconeserver.mk

@@ -29,6 +29,8 @@ SCONESERVER_CONF_OPTS += \
 ifeq ($(BR2_PACKAGE_LIBXML2),y)
 SCONESERVER_CONF_OPTS += \
 	--with-xml2-config="$(STAGING_DIR)/usr/bin/xml2-config"
+# Needed to fix build failure when icu is enabled in libxml2
+SCONESERVER_CONF_ENV += CXXFLAGS="$(TARGET_CXXFLAGS) -std=c++11"
 endif
 
 ifeq ($(BR2_PACKAGE_OPENSSL),y)