Simplify C++ flags (#17413)

"Only add c++11 flags and definitions after cuda compiling" is not true anymore

CMakeLists.txt

@@ -203,12 +203,15 @@ else()
   if(SUPPORT_CXX14)
     add_definitions(-DDMLC_USE_CXX11=1)
     add_definitions(-DDMLC_USE_CXX14=1)
+    add_definitions(-DMSHADOW_IN_CXX11)
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++14")
   elseif(SUPPORT_CXX11)
     add_definitions(-DDMLC_USE_CXX11=1)
+    add_definitions(-DMSHADOW_IN_CXX11)
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
   elseif(SUPPORT_CXX0X)
     add_definitions(-DDMLC_USE_CXX11=1)
+    add_definitions(-DMSHADOW_IN_CXX11)
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++0x")
   endif()
 endif(MSVC)
@@ -671,13 +674,7 @@ if(USE_PLUGIN_CAFFE)
   endif()
 endif()
 
-if(NOT MSVC)
-  # Only add c++11 flags and definitions after cuda compiling
-  add_definitions(-DDMLC_USE_CXX11)
-  add_definitions(-DMSHADOW_IN_CXX11)
-  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS}")
-  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++0x")
-else()
+if(MSVC)
   set(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} /EHsc")
   set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE} /EHsc /Gy")
   set(CMAKE_CXX_FLAGS_MINSIZEREL "${CMAKE_CXX_FLAGS_MINSIZEREL} /EHsc /Gy")