src,win: improve unmanaged handling

Used scoped disabling of managed code handling to ensure no other
files get affected.

PR-URL: #1852
Reviewed-By: João Reis <reis@janeasystems.com>

src/win_delay_load_hook.cc

@@ -7,10 +7,10 @@
  * This allows compiled addons to work when the host executable is renamed.
  */
 
-#pragma unmanaged
-
 #ifdef _MSC_VER
 
+#pragma managed(push, off)
+
 #ifndef WIN32_LEAN_AND_MEAN
 #define WIN32_LEAN_AND_MEAN
 #endif
@@ -34,4 +34,6 @@ static FARPROC WINAPI load_exe_hook(unsigned int event, DelayLoadInfo* info) {
 
 decltype(__pfnDliNotifyHook2) __pfnDliNotifyHook2 = load_exe_hook;
 
+#pragma managed(pop)
+
 #endif