cmake: use more portable way to enable C++11

CMakeLists.txt

@@ -152,6 +152,14 @@ if(WIN32)
   set(_gRPC_PLATFORM_WINDOWS ON)
 endif()
 
+ # Use C99 standard
+set(CMAKE_C_STANDARD 99)
+
+# Add c++11 flags
+set(CMAKE_CXX_STANDARD 11)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+set(CMAKE_CXX_EXTENSIONS OFF)
+
 set(CMAKE_POSITION_INDEPENDENT_CODE TRUE)
 set(CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake/modules")
 
@@ -201,11 +209,6 @@ include(cmake/ssl.cmake)
 include(cmake/upb.cmake)
 include(cmake/zlib.cmake)
 
-if(NOT MSVC)
-  set(CMAKE_C_FLAGS   "${CMAKE_C_FLAGS} -std=c99")
-  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
-endif()
-
 if(_gRPC_PLATFORM_MAC OR _gRPC_PLATFORM_IOS)
   set(_gRPC_ALLTARGETS_LIBRARIES ${CMAKE_DL_LIBS} m pthread)
 elseif(_gRPC_PLATFORM_ANDROID)

templates/CMakeLists.txt.template

@@ -242,6 +242,14 @@
     set(_gRPC_PLATFORM_WINDOWS ON)
   endif()
 
+   # Use C99 standard
+  set(CMAKE_C_STANDARD 99)
+
+  # Add c++11 flags
+  set(CMAKE_CXX_STANDARD 11)
+  set(CMAKE_CXX_STANDARD_REQUIRED ON)
+  set(CMAKE_CXX_EXTENSIONS OFF)
+
   ## Some libraries are shared even with BUILD_SHARED_LIBRARIES=OFF
   set(CMAKE_POSITION_INDEPENDENT_CODE TRUE)
   set(CMAKE_MODULE_PATH "<%text>${CMAKE_CURRENT_SOURCE_DIR}</%text>/cmake/modules")
@@ -292,11 +300,6 @@
   include(cmake/upb.cmake)
   include(cmake/zlib.cmake)
 
-  if(NOT MSVC)
-    set(CMAKE_C_FLAGS   "<%text>${CMAKE_C_FLAGS}</%text> -std=c99")
-    set(CMAKE_CXX_FLAGS "<%text>${CMAKE_CXX_FLAGS}</%text> -std=c++11")
-  endif()
-
   if(_gRPC_PLATFORM_MAC OR _gRPC_PLATFORM_IOS)
     set(_gRPC_ALLTARGETS_LIBRARIES <%text>${CMAKE_DL_LIBS}</%text> m pthread)
   elseif(_gRPC_PLATFORM_ANDROID)