Use _WIN32 instead of WIN32 to detect Windows

`WIN32` is not defined when building a 64-bit executable on Windows with Clang. `_WIN32` is the more appropriate option here.
http://web.archive.org/web/20191012035921/http://nadeausoftware.com/articles/2012/01/c_c_tip_how_use_compiler_predefined_macros_detect_operating_system contains a handy table/matrix of the options and why this is best.

rtgui/batchqueue.cc

@@ -253,7 +253,7 @@ bool BatchQueue::saveBatchQueue ()
             const auto& saveFormat = entry->saveFormat;
 
             // Warning: for code's simplicity in loadBatchQueue, each field must end by the '|' character, safer than ';' or ',' since it can't be used in paths
-#ifdef WIN32
+#ifdef _WIN32
             // on windows it crashes if we don't use c_str() and filename etc. contain special (e.g. chinese) characters, see issue 3387
             file << entry->filename.c_str() << '|' << entry->savedParamsFile.c_str() << '|' << entry->outFileName.c_str() << '|' << saveFormat.format << '|'
 #else