Fix cmake on macos (#32241)

CMakeLists.txt

@@ -229,6 +229,12 @@ if(WIN32)
   set(_gRPC_PLATFORM_WINDOWS ON)
 endif()
 
+if (APPLE AND NOT DEFINED CMAKE_CXX_STANDARD)
+  # AppleClang defaults to C++98, so we bump it to C++14.
+  message("CMAKE_CXX_STANDARD was undefined, defaulting to C++14.")
+  set(CMAKE_CXX_STANDARD 14)
+endif ()
+
 if (NOT DEFINED CMAKE_POSITION_INDEPENDENT_CODE)
   set(CMAKE_POSITION_INDEPENDENT_CODE TRUE)
 endif()

templates/CMakeLists.txt.template

@@ -264,6 +264,12 @@
     set(_gRPC_PLATFORM_WINDOWS ON)
   endif()
 
+  if (APPLE AND NOT DEFINED CMAKE_CXX_STANDARD)
+    # AppleClang defaults to C++98, so we bump it to C++14.
+    message("CMAKE_CXX_STANDARD was undefined, defaulting to C++14.")
+    set(CMAKE_CXX_STANDARD 14)
+  endif ()
+
   ## Some libraries are shared even with BUILD_SHARED_LIBRARIES=OFF
   if (NOT DEFINED CMAKE_POSITION_INDEPENDENT_CODE)
     set(CMAKE_POSITION_INDEPENDENT_CODE TRUE)

tools/run_tests/run_tests.py

@@ -477,7 +477,7 @@ class CLanguage(object):
             _check_compiler(compiler, ['default', 'cmake'])
 
         if compiler == 'default' or compiler == 'cmake':
-            return ('debian11', ["-DCMAKE_CXX_STANDARD=14"])
+            return ('debian11', [])
         elif compiler == 'gcc7':
             return ('gcc_7', [])
         elif compiler == 'gcc10.2':