[CMake] Don't change CMAKE_REQUIRED_FLAGS when permanently setting -std=...

CMAKE_REQUIRED_FLAGS is explicitly designed for temporary flags used in
CMake's various check_<something> macros, so don't store any test
outcome in there.

cmake/modules/CheckCXXFeatures.cmake

@@ -63,7 +63,6 @@ if(NOT DISABLE_CXX_VERSION_CHECK)
 endif()
 
 if(CXX_FLAG_CXX14 AND CXX_LIB_SUPPORTS_CXX14)
-  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -std=c++14")
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++14 ")
   set(CXX_STD14_FLAGS "-std=c++14")
 else()
@@ -77,7 +76,6 @@ else()
     cmake_pop_check_state()
   endif()
   if(CXX_FLAG_CXX1Y AND CXX_LIB_SUPPORTS_CXX1Y)
-    set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -std=c++1y" )
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++1y ")
   set(CXX_STD14_FLAGS "-std=c++1y")
   endif()
@@ -91,7 +89,6 @@ if(NOT DISABLE_CXX_VERSION_CHECK
   check_cxx_compiler_flag("-std=c++11" CXX_FLAG_CXX11)
 
   if(CXX_FLAG_CXX11)
-    set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -std=c++11")
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11 ")
     set(CXX_STD11_FLAGS "-std=c++11")
   else()