configure.ac: fix grpc build

configure.ac: fix grpc build

abseil and other google tools are now subject to
"Google's Foundational C++ Support Policy" [0][1]. This currently
mandates gcc 7.3.1 and C++14 as minimum versions. So replace -std=c++11
by -std=c++14 to fix following build failure:

checking whether /home/buildroot/autobuild/instance-0/output-1/host/bin/sparc64-linux-g++ accepts -std=c++11
configure:25105: result: yes
configure:25127: checking grpc++/grpc++.h usability
configure:25127: /home/buildroot/autobuild/instance-0/output-1/host/bin/sparc64-linux-g++ -c -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64  -O1 -g0 -D_FORTIFY_SOURCE=2 -std=c++11  -DNOMINMAX  -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 conftest.cpp >&5
In file included from /home/buildroot/autobuild/instance-0/output-1/host/sparc64-buildroot-linux-gnu/sysroot/usr/include/absl/base/config.h:86,
                 from /home/buildroot/autobuild/instance-0/output-1/host/sparc64-buildroot-linux-gnu/sysroot/usr/include/absl/base/const_init.h:25,
                 from /home/buildroot/autobuild/instance-0/output-1/host/sparc64-buildroot-linux-gnu/sysroot/usr/include/absl/synchronization/mutex.h:67,
                 from /home/buildroot/autobuild/instance-0/output-1/host/sparc64-buildroot-linux-gnu/sysroot/usr/include/grpcpp/impl/codegen/sync.h:32,
                 from /home/buildroot/autobuild/instance-0/output-1/host/sparc64-buildroot-linux-gnu/sysroot/usr/include/grpcpp/completion_queue.h:41,
                 from /home/buildroot/autobuild/instance-0/output-1/host/sparc64-buildroot-linux-gnu/sysroot/usr/include/grpcpp/channel.h:25,
                 from /home/buildroot/autobuild/instance-0/output-1/host/sparc64-buildroot-linux-gnu/sysroot/usr/include/grpcpp/grpcpp.h:52,
                 from /home/buildroot/autobuild/instance-0/output-1/host/sparc64-buildroot-linux-gnu/sysroot/usr/include/grpc++/grpc++.h:26,
                 from conftest.cpp:167:
/home/buildroot/autobuild/instance-0/output-1/host/sparc64-buildroot-linux-gnu/sysroot/usr/include/absl/base/policy_checks.h:79:2: error: #error "C++ versions less than C++14 are not supported."
   79 | #error "C++ versions less than C++14 are not supported."
      |  ^~~~~

[0] https://github.com/abseil/abseil-cpp/releases/tag/20230125.0
[1] https://github.com/google/oss-policies-info/blob/b842c39db88e6569dfe2cf98be434b03507cb503/foundational-cxx-support-matrix.md

Fixes:
 - http://autobuild.buildroot.org/results/e2cf909ab3c356f1881fd18cfa039e87faa10d7d

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