CMake: automatically handle C++11 settings if possible

CMakeLists.txt

@@ -31,6 +31,10 @@ option(RAPIDJSON_BUILD_THIRDPARTY_GTEST
     "Use gtest installation in `thirdparty/gtest` by default if available" OFF)
 
 option(RAPIDJSON_BUILD_CXX11 "Build rapidjson with C++11 (gcc/clang)" ON)
+if(RAPIDJSON_BUILD_CXX11)
+    set(CMAKE_CXX_STANDARD 11)
+    set(CMAKE_CXX_STANDARD_REQUIRED TRUE)
+endif()
 
 option(RAPIDJSON_BUILD_ASAN "Build rapidjson with address sanitizer (gcc/clang)" OFF)
 option(RAPIDJSON_BUILD_UBSAN "Build rapidjson with undefined behavior sanitizer (gcc/clang)" OFF)
@@ -62,7 +66,7 @@ if (CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
     endif()
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wextra -Werror")
     set(EXTRA_CXX_FLAGS -Weffc++ -Wswitch-default -Wfloat-equal -Wconversion -Wsign-conversion)
-    if (RAPIDJSON_BUILD_CXX11)
+    if (RAPIDJSON_BUILD_CXX11 AND CMAKE_VERSION VERSION_LESS 3.1)
         if (CMAKE_CXX_COMPILER_VERSION VERSION_LESS "4.7.0")
             set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++0x")
         else()
@@ -94,7 +98,7 @@ elseif (CMAKE_CXX_COMPILER_ID MATCHES "Clang")
     endif()
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wextra -Werror -Wno-missing-field-initializers")
     set(EXTRA_CXX_FLAGS -Weffc++ -Wswitch-default -Wfloat-equal -Wconversion -Wimplicit-fallthrough)
-    if (RAPIDJSON_BUILD_CXX11)
+    if (RAPIDJSON_BUILD_CXX11 AND CMAKE_VERSION VERSION_LESS 3.1)
         set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
     endif()
     if (RAPIDJSON_BUILD_ASAN)