Use _WIN32 macro for detecting MSVC Windows

Previously we used `WIN32`, which sometimes does exist, but according to
https://docs.microsoft.com/en-us/cpp/preprocessor/predefined-macros?view=vs-2019
`_WIN32` is the one that is actually provided by the compiler.

Closes #1676.

Author: horenmar

include/internal/catch_session.cpp

@@ -197,7 +197,7 @@ namespace Catch {
         return 0;
     }
 
-#if defined(CATCH_CONFIG_WCHAR) && defined(WIN32) && defined(UNICODE)
+#if defined(CATCH_CONFIG_WCHAR) && defined(_WIN32) && defined(UNICODE)
     int Session::applyCommandLine( int argc, wchar_t const * const * argv ) {
 
         char **utf8Argv = new char *[ argc ];

include/internal/catch_session.h

@@ -26,7 +26,7 @@ namespace Catch {
         void libIdentify();
 
         int applyCommandLine( int argc, char const * const * argv );
-    #if defined(CATCH_CONFIG_WCHAR) && defined(WIN32) && defined(UNICODE)
+    #if defined(CATCH_CONFIG_WCHAR) && defined(_WIN32) && defined(UNICODE)
         int applyCommandLine( int argc, wchar_t const * const * argv );
     #endif