[CMake] Don't pollute build-specific CXX_FLAGS with global compiler flags

The C++ version flag check used to set the -std=... flag for both the
basic CMAKE_CXX_FLAGS and the build-specific variants of it. That is
unnecessary because CMAKE concatenates the base variable and the
build-specific variant (if a build type has been set by the
user). Having both is actually rather unhelpful, as any downstream
changes will also have to be done to the complete set of variables.

cmake/modules/CheckCXXFeatures.cmake

@@ -65,10 +65,6 @@ 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(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} -std=c++14 ")
-  set(CMAKE_CXX_FLAGS_MINSIZEREL "${CMAKE_CXX_FLAGS_MINSIZEREL} -std=c++14 ")
-  set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE} -std=c++14 ")
-  set(CMAKE_CXX_FLAGS_RELWITHDEBINFO "${CMAKE_CXX_FLAGS_RELWITHDEBINFO} -std=c++14 ")
   set(CXX_STD14_FLAGS "-std=c++14")
 else()
   if(NOT DISABLE_CXX_VERSION_CHECK)
@@ -83,10 +79,6 @@ else()
   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(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} -std=c++1y ")
-    set(CMAKE_CXX_FLAGS_MINSIZEREL "${CMAKE_CXX_FLAGS_MINSIZEREL} -std=c++1y ")
-    set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE} -std=c++1y ")
-    set(CMAKE_CXX_FLAGS_RELWITHDEBINFO "${CMAKE_CXX_FLAGS_RELWITHDEBINFO} -std=c++1y ")
   set(CXX_STD14_FLAGS "-std=c++1y")
   endif()
 endif()
@@ -101,10 +93,6 @@ if(NOT DISABLE_CXX_VERSION_CHECK
   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(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} -std=c++11 ")
-    set(CMAKE_CXX_FLAGS_MINSIZEREL "${CMAKE_CXX_FLAGS_MINSIZEREL} -std=c++11 ")
-    set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE} -std=c++11 ")
-    set(CMAKE_CXX_FLAGS_RELWITHDEBINFO "${CMAKE_CXX_FLAGS_RELWITHDEBINFO} -std=c++11 ")
     set(CXX_STD11_FLAGS "-std=c++11")
   else()
     message(FATAL_ERROR "Your compiler does not seem to support C++11. If it does, please add any required flags to your CMAKE_CXX_FLAGS or set DISABLE_CXX_VERSION_CHECK.")