Windows : small cmake cleanup

Use STRING(APPEND VAR...) rather than SET(VAR "${VAR} ..."), it is shorter

cmake/os/Windows.cmake

@@ -201,25 +201,25 @@ IF(MSVC)
   # Mark 32 bit executables large address aware so they can 
   # use > 2GB address space
   IF(CMAKE_SIZEOF_VOID_P MATCHES 4)
-    SET(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} /LARGEADDRESSAWARE")
+   STRING(APPEND CMAKE_EXE_LINKER_FLAGS " /LARGEADDRESSAWARE")
   ENDIF()
   
   # Speed up multiprocessor build
   IF (NOT CLANG_CL)
-    SET(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} /MP")
-    SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /MP")
+    STRING(APPEND CMAKE_C_FLAGS " /MP")
+    STRING(APPEND CMAKE_CXX_FLAGS " /MP")
   ENDIF()
   
   #TODO: update the code and remove the disabled warnings
-  SET(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} /we4700 /we4311 /we4477 /we4302 /we4090")
-  SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /we4099 /we4700 /we4311 /we4477 /we4302 /we4090")
+  STRING(APPEND CMAKE_C_FLAGS " /we4700 /we4311 /we4477 /we4302 /we4090")
+  STRING(APPEND CMAKE_CXX_FLAGS " /we4099 /we4700 /we4311 /we4477 /we4302 /we4090")
   IF(MSVC_VERSION GREATER 1910  AND NOT CLANG_CL)
-    SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /permissive-")
+    STRING(APPEND CMAKE_CXX_FLAGS " /permissive-")
   ENDIF()
   ADD_DEFINITIONS(-D_CRT_NONSTDC_NO_WARNINGS)
   IF(MYSQL_MAINTAINER_MODE MATCHES "ERR")
-    SET(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} /WX")
-    SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /WX")
+    STRING(APPEND CMAKE_C_FLAGS " /WX")
+    STRING(APPEND CMAKE_CXX_FLAGS " /WX")
     FOREACH(type EXE SHARED MODULE)
       FOREACH(cfg RELEASE DEBUG RELWITHDEBINFO)
         SET(CMAKE_${type}_LINKER_FLAGS_${cfg} "${CMAKE_${type}_LINKER_FLAGS_${cfg}} /WX")
@@ -229,9 +229,9 @@ IF(MSVC)
   IF(MSVC_VERSION LESS 1910)
     # Noisy warning C4800: 'type': forcing value to bool 'true' or 'false' (performance warning),
     # removed in VS2017
-    SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /wd4800")
+    STRING(APPEND CMAKE_CXX_FLAGS " /wd4800")
   ELSEIF (NOT CLANG_CL)
-    SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /d2OptimizeHugeFunctions")
+    STRING(APPEND CMAKE_CXX_FLAGS " /d2OptimizeHugeFunctions")
   ENDIF()
 ENDIF()