build: correctly set MSVC __cplusplus (#8120)

* build: correctly set MSVC `__cplusplus`

* refactor: let utf8cpp infer `UTF_CPP_CPLUSPLUS`

CMakeLists.txt

@@ -223,6 +223,8 @@ if(WIN32)
             set(CMAKE_${L}_FLAGS "${CMAKE_${L}_FLAGS} /utf-8")
             # Reduce noise (at least for now)
             set(CMAKE_${L}_FLAGS "${CMAKE_${L}_FLAGS} /wd4244 /wd4267")
+            # Correctly set __cplusplus macro value
+            set(CMAKE_${L}_FLAGS "${CMAKE_${L}_FLAGS} /Zc:__cplusplus")
             # Make caching-friendly (store debug info inside object files)
             foreach(T IN ITEMS ${CMAKE_CONFIGURATION_TYPES} ${CMAKE_BUILD_TYPE})
                 string(TOUPPER "${T}" T)

libtransmission/utils.cc

@@ -36,7 +36,6 @@
 #include <sys/stat.h> /* umask() */
 #endif
 
-#define UTF_CPP_CPLUSPLUS 201703L
 #include <utf8.h>
 
 #include <curl/curl.h>