configure.ac: fix protobuf/util/time_util.h detection

configure.ac: fix protobuf/util/time_util.h detection

Add -std=c++11 when checking gor protobuf/util/time_util.h otherwise
test will fail on:

configure:104495: checking google/protobuf/util/time_util.h usability
configure:104495: /home/br-user/autobuild/run/instance-2/output/host/bin/mips-linux-gnu-g++ -c -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64  -Os  -D_FORTIFY_SOURCE=1  -pthread  conftest.cpp >&5
In file included from /home/br-user/autobuild/run/instance-2/output/host/opt/ext-toolchain/mips-linux-gnu/include/c++/5.3.0/atomic:38:0,
                 from /home/br-user/autobuild/run/instance-2/output/host/mips-buildroot-linux-gnu/sysroot/usr/include/google/protobuf/io/coded_stream.h:113,
                 from /home/br-user/autobuild/run/instance-2/output/host/mips-buildroot-linux-gnu/sysroot/usr/include/google/protobuf/duration.pb.h:23,
                 from /home/br-user/autobuild/run/instance-2/output/host/mips-buildroot-linux-gnu/sysroot/usr/include/google/protobuf/util/time_util.h:52,
                 from conftest.cpp:188:
/home/br-user/autobuild/run/instance-2/output/host/opt/ext-toolchain/mips-linux-gnu/include/c++/5.3.0/bits/c++0x_warning.h:32:2: error: #error This file requires compiler and library support for the ISO C++ 2011 standard. This support is currently experimental, and must be enabled with the -std=c++11 or -std=gnu++11 compiler options.

Fixes:
 - http://autobuild.buildroot.net/results/0d486edaf4c51b6438adeec61fe2c55099862acb

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>