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 <[email protected]>

configure.ac

@@ -2853,20 +2853,20 @@ PKG_CHECK_MODULES([GRPCPP], [grpc++],
 fi
 
 if test "x$withval" != "xno"; then
-  AC_MSG_CHECKING([whether $CXX accepts -std=c++11])
-  if test_cxx_flags -std=c++11; then
+  AC_MSG_CHECKING([whether $CXX accepts -std=c++14])
+  if test_cxx_flags -std=c++14; then
     AC_MSG_RESULT([yes])
   else
     AC_MSG_RESULT([no])
-    with_libgrpcpp="no (requires C++11 support)"
-    with_libprotobuf="no (<google/protobuf/util/time_util.h> requires C++11 support)"
+    with_libgrpcpp="no (requires C++14 support)"
+    with_libprotobuf="no (<absl/base/policy_checks.h> requires C++14 support)"
   fi
 fi
 
 if test "x$with_libgrpcpp" = "xyes"; then
   AC_LANG_PUSH(C++)
   SAVE_CPPFLAGS="$CPPFLAGS"
-  CPPFLAGS="-std=c++11 $with_libgrpcpp_cppflags $GRPCPP_CFLAGS $CPPFLAGS"
+  CPPFLAGS="-std=c++14 $with_libgrpcpp_cppflags $GRPCPP_CFLAGS $CPPFLAGS"
   AC_CHECK_HEADERS([grpc++/grpc++.h],
     [with_libgrpcpp="yes"],
     [with_libgrpcpp="no (<grpc++/grpc++.h> not found)"]
@@ -2880,7 +2880,7 @@ if test "x$with_libgrpcpp" = "xyes"; then
   SAVE_CPPFLAGS="$CPPFLAGS"
   SAVE_LDFLAGS="$LDFLAGS"
   SAVE_LIBS="$LIBS"
-  CPPFLAGS="-std=c++11 $with_libgrpcpp_cppflags $GRPCPP_CFLAGS $CPPFLAGS"
+  CPPFLAGS="-std=c++14 $with_libgrpcpp_cppflags $GRPCPP_CFLAGS $CPPFLAGS"
   LDFLAGS="$with_libgrpcpp_ldflags"
   if test "x$GRPCPP_LIBS" = "x"; then
     LIBS="-lgrpc++"
@@ -2908,7 +2908,7 @@ if test "x$with_libgrpcpp" = "xyes"; then
   AC_LANG_POP(C++)
 fi
 
-BUILD_WITH_LIBGRPCPP_CPPFLAGS="-std=c++11 $with_libgrpcpp_cppflags $GRPCPP_CFLAGS"
+BUILD_WITH_LIBGRPCPP_CPPFLAGS="-std=c++14 $with_libgrpcpp_cppflags $GRPCPP_CFLAGS"
 BUILD_WITH_LIBGRPCPP_LDFLAGS="$with_libgrpcpp_ldflags"
 BUILD_WITH_LIBGRPCPP_LIBS="$GRPCPP_LIBS"
 AC_SUBST([BUILD_WITH_LIBGRPCPP_CPPFLAGS])
@@ -4852,7 +4852,7 @@ if test "x$withval" != "xno"; then
   AC_CHECK_LIB([protobuf], [main],
     [
       SAVE_CPPFLAGS="$CPPFLAGS"
-      CPPFLAGS="-std=c++11 $with_libprotobuf_cppflags $PROTOBUF_CFLAGS"
+      CPPFLAGS="-std=c++14 $with_libprotobuf_cppflags $PROTOBUF_CFLAGS"
       if test "x$PROTOBUF_LIBS" = "x"
       then
         PROTOBUF_LIBS="-lprotobuf"